can rays travel through vacuum

Anatomy of an Electromagnetic Wave

Energy, a measure of the ability to do work, comes in many forms and can transform from one type to another. Examples of stored or potential energy include batteries and water behind a dam. Objects in motion are examples of kinetic energy. Charged particles—such as electrons and protons—create electromagnetic fields when they move, and these fields transport the type of energy we call electromagnetic radiation, or light.

What are Electromagnetic and Mechanical waves?

Mechanical waves and electromagnetic waves are two important ways that energy is transported in the world around us. Waves in water and sound waves in air are two examples of mechanical waves. Mechanical waves are caused by a disturbance or vibration in matter, whether solid, gas, liquid, or plasma. Matter that waves are traveling through is called a medium. Water waves are formed by vibrations in a liquid and sound waves are formed by vibrations in a gas (air). These mechanical waves travel through a medium by causing the molecules to bump into each other, like falling dominoes transferring energy from one to the next. Sound waves cannot travel in the vacuum of space because there is no medium to transmit these mechanical waves.

ELECTROMAGNETIC WAVES

Electricity can be static, like the energy that can make your hair stand on end. Magnetism can also be static, as it is in a refrigerator magnet. A changing magnetic field will induce a changing electric field and vice-versa—the two are linked. These changing fields form electromagnetic waves. Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.

In the 1860's and 1870's, a Scottish scientist named James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as "Maxwell's Equations."

Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. The unit of frequency of a radio wave -- one cycle per second -- is named the hertz, in honor of Heinrich Hertz.

His experiment with radio waves solved two problems. First, he had demonstrated in the concrete, what Maxwell had only theorized — that the velocity of radio waves was equal to the velocity of light! This proved that radio waves were a form of light! Second, Hertz found out how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell's waves — electromagnetic waves.

WAVES OR PARTICLES? YES!

Light is made of discrete packets of energy called photons. Photons carry momentum, have no mass, and travel at the speed of light. All light has both particle-like and wave-like properties. How an instrument is designed to sense the light influences which of these properties are observed. An instrument that diffracts light into a spectrum for analysis is an example of observing the wave-like property of light. The particle-like nature of light is observed by detectors used in digital cameras—individual photons liberate electrons that are used for the detection and storage of the image data.

POLARIZATION

One of the physical properties of light is that it can be polarized. Polarization is a measurement of the electromagnetic field's alignment. In the figure above, the electric field (in red) is vertically polarized. Think of a throwing a Frisbee at a picket fence. In one orientation it will pass through, in another it will be rejected. This is similar to how sunglasses are able to eliminate glare by absorbing the polarized portion of the light.

DESCRIBING ELECTROMAGNETIC ENERGY

The terms light, electromagnetic waves, and radiation all refer to the same physical phenomenon: electromagnetic energy. This energy can be described by frequency, wavelength, or energy. All three are related mathematically such that if you know one, you can calculate the other two. Radio and microwaves are usually described in terms of frequency (Hertz), infrared and visible light in terms of wavelength (meters), and x-rays and gamma rays in terms of energy (electron volts). This is a scientific convention that allows the convenient use of units that have numbers that are neither too large nor too small.

The number of crests that pass a given point within one second is described as the frequency of the wave. One wave—or cycle—per second is called a Hertz (Hz), after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second has a frequency of 2 Hz.

Electromagnetic waves have crests and troughs similar to those of ocean waves. The distance between crests is the wavelength. The shortest wavelengths are just fractions of the size of an atom, while the longest wavelengths scientists currently study can be larger than the diameter of our planet!

An electromagnetic wave can also be described in terms of its energy—in units of measure called electron volts (eV). An electron volt is the amount of kinetic energy needed to move an electron through one volt potential. Moving along the spectrum from long to short wavelengths, energy increases as the wavelength shortens. Consider a jump rope with its ends being pulled up and down. More energy is needed to make the rope have more waves.

Next: Wave Behaviors

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Anatomy of an Electromagnetic Wave. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/02_anatomy

Science Mission Directorate. "Anatomy of an Electromagnetic Wave" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/02_anatomy

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How Light Travels: The Reason Why Telescopes Can See the Invisible Parts of Our Universe

Due to how light travels, we can only see the most eye-popping details of space—like nebulas, supernovas, and black holes—with specialized telescopes.

• Our eyes can see only a tiny fraction of these wavelengths , but our instruments enable us to learn far more.
• Here, we outline how various telescopes detect different wavelengths of light from space.

Light travels only one way: in a straight line. But the path it takes from Point A to Point B is always a waveform, with higher-energy light traveling in shorter wavelengths. Photons , which are tiny parcels of energy, have been traveling across the universe since they first exploded from the Big Bang . They always travel through the vacuum of space at 186,400 miles per second—the speed of light—which is faster than anything else.

Too bad we can glimpse only about 0.0035 percent of the light in the universe with our naked eyes. Humans can perceive just a tiny sliver of the electromagnetic spectrum: wavelengths from about 380–750 nanometers. This is what we call the visible part of the electromagnetic spectrum. The universe may be lovely to look at in this band, but our vision skips right over vast ranges of wavelengths that are either shorter or longer than this limited range. On either side of the visible band lies evidence of interstellar gas clouds, the hottest stars in the universe, gas clouds between galaxies , the gas that rushes into black holes, and much more.

Fortunately, telescopes allow us to see what would otherwise remain hidden. To perceive gas clouds between stars and galaxies, we use detectors that can capture infrared wavelengths. Super-hot stars require instruments that see short, ultraviolet wavelengths. To see the gas clouds between galaxies, we need X-ray detectors.

We’ve been using telescopes designed to reveal the invisible parts of the cosmos for more than 60 years. Because Earth’s atmosphere absorbs most wavelengths of light, many of our telescopes must observe the cosmos from orbit or outer space.

Here’s a snapshot of how we use specialized detectors to explore how light travels across the universe.

Infrared Waves

We can’t see infrared waves, but we can feel them as heat . A sensitive detector like the James Webb Space Telescope can discern this thermal energy from far across the universe. But we use infrared in more down-to-Earth ways as well. For example, remote-control devices work by sending infrared signals at about 940 nanometers to your television or stereo. These heat waves also emanate from incubators to help hatch a chick or keep a pet reptile warm. As a warm being, you radiate infrared waves too; a person using night vision goggles can see you, because the goggles turn infrared energy into false-color optical energy that your eyes can perceive. Infrared telescopes let us see outer space in a similar way.

Astronomers began the first sky surveys with infrared telescopes in the 1960s and 1970s. Webb , launched in 2021, takes advantage of the infrared spectrum to probe the deepest regions of the universe. Orbiting the sun at a truly cold expanse—about one million miles from Earth—Webb has three infrared detectors with the ability to peer farther back in time than any other telescope has so far.

Its primary imaging device, the Near Infrared Camera (NIRCam), observes the universe through detectors tuned to incoming wavelengths ranging from 0.6 to 5 microns, ideal for seeing light from the universe’s earliest stars and galaxies. Webb’s Mid-Infrared Instrument (MIRI) covers the wavelength range from 5 to 28 microns, its sensitive detectors collecting the redshifted light of distant galaxies. Conveniently for us, infrared passes more cleanly through deep space gas and dust clouds, revealing the objects behind them; for this and many other reasons, the infrared spectrum has gained a crucial foothold in our cosmic investigations. Earth-orbiting satellites like NASA’s Wide Field Infrared Survey Telescope ( WFIRST ) observe deep space via longer infrared wavelengths, too.

Yet, when stars first form, they mostly issue ultraviolet light . So why don’t we use ultraviolet detectors to find distant galaxies? It’s because the universe has been stretching since its beginning, and the light that travels through it has been stretching, too; every planet, star, and galaxy continually moves away from everything else. By the time light from GLASS-z13—formed 300 million years after the Big Bang—reaches our telescopes, it has been traveling for more than 13 billion years , a vast distance all the way from a younger universe. The light may have started as ultraviolet waves, but over vast scales of time and space, it ended up as infrared. So, this fledgling galaxy appears as a red dot to NIRCam. We are gazing back in time at a galaxy that is rushing away from us.

Radio Waves

If we could see the night sky only through radio waves, we would notice swaths of supernovae , pulsars, quasars, and gassy star-forming regions instead of the usual pinprick fairy lights of stars and planets.

Tools like the Arecibo Observatory in Puerto Rico can do the job our eyes can’t: detect some of the longest electromagnetic waves in the universe. Radio waves are typically the length of a football field, but they can be even longer than our planet’s diameter. Though the 1,000-foot-wide dish at Arecibo collapsed in 2020 due to structural problems, other large telescopes carry on the work of looking at radio waves from space. Large radio telescopes are special because they actually employ many smaller dishes, integrating their data to produce a really sharp image.

Unlike optical astronomy, ground-based radio telescopes don’t need to contend with clouds and rain. They can make out the composition, structure, and motion of planets and stars no matter the weather. However, the dishes of radio telescopes need to be much larger than optical ones to generate a comparable image, since radio waves are so long. The Parkes Observatory’s dish is 64 meters wide, but its imaging is comparable to a small backyard optical telescope, according to NASA .

Eight different radio telescopes all over the world coordinated their observations for the Event Horizon Telescope in 2019 to put together the eye-opening image of a black hole in the heart of the M87 galaxy (above).

Ultraviolet Waves

You may be most familiar with ultraviolet, or UV rays, in warnings to use sunscreen . The sun is our greatest local emitter of these higher-frequency, shorter wavelengths just beyond the human visible spectrum, ranging from 100 to 400 nanometers. The Hubble Space Telescope has been our main instrument for observing UV light from space, including young stars forming in Spiral Galaxy NGC 3627, the auroras of Jupiter, and a giant cloud of hydrogen evaporating from an exoplanet that is reacting to its star’s extreme radiation.

Our sun and other stars emit a full range of UV light, telling astronomers how relatively hot or cool they are according to the subdivisions of ultraviolet radiation: near ultraviolet, middle ultraviolet, far ultraviolet, and extreme ultraviolet. Applying a false-color visible light composite lets us see with our own eyes the differences in a star’s gas temperatures.

Hubble’s Wide Field Camera 3 (WFC3) breaks down ultraviolet light into specific present colors with filters. “Science visuals developers assign primary colors and reconstruct the data into a picture our eyes can clearly identify,” according to the Hubble website . Using image-processing software, astronomers and even amateur enthusiasts can turn the UV data into images that are not only beautiful, but also informative.

X-Ray Light

Since 1999, the orbiting Chandra X-Ray Observatory is the most sensitive radio telescope ever built. During one observation that lasted a few hours, its X-ray vision saw only four photons from a galaxy 240 million light-years away, but it was enough to ascertain a novel type of exploding star . The observatory, located 86,500 miles above Earth, can produce detailed, full-color images of hot X-ray-emitting objects, like supernovas, clusters of galaxies and gases, and jets of energy surrounding black holes that are millions of degrees Celsius. It can also measure the intensity of an individual X-ray wavelength, which ranges from just 0.01 to 10 nanometers. Its four sensitive mirrors pick up energetic photons and then electronic detectors at the end of a 30-foot optical apparatus focus the beams of X-rays.

Closer to home, the Aurora Borealis at the poles emits X-rays too. And down on Earth, this high-frequency, low-wavelength light passes easily through the soft tissue of our bodies, but not our bones, yielding stellar X-ray images of our skeletons and teeth.

Visible Light

Visible color gives astronomers essential clues to a whole world of information about a star, including temperature, distance, mass, and chemical composition. The Hubble Telescope, perched 340 miles above our planet, has been a major source of visible light images of the cosmos since 1990.

Hotter objects, like young stars, radiate energy at shorter wavelengths of light; that’s why younger stars at temperatures up to 12,000 degrees Celsius, like the star Rigel, look blue to us. Astronomers can also tell the mass of a star from its color. Because mass corresponds to temperature, observers know that hot blue stars are at least three times the mass of the sun. For instance, the extremely hot, luminous blue variable star Eta Carina’s bulk is 150 times the mass of our sun, and it radiates 1,000,000 times our sun’s energy.

Our comparatively older, dimmer sun is about 5,500 degrees Celsius, so it appears yellow. At the other end of the scale, the old star Betelgeuse has been blowing off its outer layer for the past few years, and it looks red because it’s only about 3,000 degrees Celsius.

A View of Earth

Scientists use different wavelengths of light to study phenomena closer to home, too.

Detectors in orbit can distinguish between geophysical and environmental features on Earth’s changing surface, such as volcanic action. For example, infrared light used alongside visible light detection reveals areas covered in snow, volcanic ash, and vegetation. The Moderate Resolution Imaging Spectroradiometer ( MODIS ) infrared instrument onboard the Aqua and Terra satellites monitors forest fire smoke and locates the source of a fire so humans don’t have to fly through smoke to evaluate the situation.

Next year, a satellite will be launched to gauge forest biomass using a special radar wavelength of about 70 centimeters that can penetrate the leafy canopy.

💡 Why is the sky blue? During the day, oxygen and nitrogen in Earth’s atmosphere scatters electromagnetic energy at the wavelengths of blue light (450–485 nanometers). At sunset, the sun’s light makes a longer journey through the atmosphere before greeting your eyes. Along the way, more of the sun’s light is scattered out of the blue spectrum and deeper into yellow and red.

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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May 20, 2016

How does light travel?

by Matt Williams, Universe Today

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century's BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern, the 20th century led to breakthroughs that showed that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein's Theory of Relativity. However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light in the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle's theory of light, which viewed it as being a disturbance in the air (one of his four "elements" that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or "corpuscles"). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.

Newton's corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise "Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light". According to Newton, the principles of light could be summed as follows:

• Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
• These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to "wave theory", which had been advocated by 17th century Dutch astronomer Christiaan Huygens. . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his "Traité de la lumière" ("Treatise on Light"). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment, which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young's version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation , would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter's moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century , James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell's equations) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c).

In 1905, Albert Einstein published "On the Electrodynamics of Moving Bodies", in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity, which reconciled Maxwell's equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect, Einstein based his idea on Planck's earlier work with "black bodies" – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein's photoelectric effect was attempt to explain the "black body problem", in which a black body emits electromagnetic radiation due to the object's heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein's explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named "photons". For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg's "uncertainty principle" (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger's paradox that claimed that all particles have a " wave function ".

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly "collapse", or rather "decohere", to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the "Schrödinger's Cat" paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly slow down or arrest the speed of light is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

Source: Universe Today

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16.1 Traveling Waves

Learning objectives.

By the end of this section, you will be able to:

• Describe the basic characteristics of wave motion
• Define the terms wavelength, amplitude, period, frequency, and wave speed
• Explain the difference between longitudinal and transverse waves, and give examples of each type
• List the different types of waves

We saw in Oscillations that oscillatory motion is an important type of behavior that can be used to model a wide range of physical phenomena. Oscillatory motion is also important because oscillations can generate waves, which are of fundamental importance in physics. Many of the terms and equations we studied in the chapter on oscillations apply equally well to wave motion ( Figure 16.2 ).

Types of Waves

A wave is a disturbance that propagates, or moves from the place it was created. There are three basic types of waves: mechanical waves, electromagnetic waves, and matter waves.

Basic mechanical wave s are governed by Newton’s laws and require a medium. A medium is the substance mechanical waves propagate through, and the medium produces an elastic restoring force when it is deformed. Mechanical waves transfer energy and momentum, without transferring mass. Some examples of mechanical waves are water waves, sound waves, and seismic waves. The medium for water waves is water; for sound waves, the medium is usually air. (Sound waves can travel in other media as well; we will look at that in more detail in Sound .) For surface water waves, the disturbance occurs on the surface of the water, perhaps created by a rock thrown into a pond or by a swimmer splashing the surface repeatedly. For sound waves, the disturbance is a change in air pressure, perhaps created by the oscillating cone inside a speaker or a vibrating tuning fork. In both cases, the disturbance is the oscillation of the molecules of the fluid. In mechanical waves, energy and momentum transfer with the motion of the wave, whereas the mass oscillates around an equilibrium point. (We discuss this in Energy and Power of a Wave .) Earthquakes generate seismic waves from several types of disturbances, including the disturbance of Earth’s surface and pressure disturbances under the surface. Seismic waves travel through the solids and liquids that form Earth. In this chapter, we focus on mechanical waves.

Electromagnetic waves are associated with oscillations in electric and magnetic fields and do not require a medium. Examples include gamma rays, X-rays, ultraviolet waves, visible light, infrared waves, microwaves, and radio waves. Electromagnetic waves can travel through a vacuum at the speed of light, v = c = 2.99792458 × 10 8 m/s . v = c = 2.99792458 × 10 8 m/s . For example, light from distant stars travels through the vacuum of space and reaches Earth. Electromagnetic waves have some characteristics that are similar to mechanical waves; they are covered in more detail in Electromagnetic Waves .

Matter waves are a central part of the branch of physics known as quantum mechanics. These waves are associated with protons, electrons, neutrons, and other fundamental particles found in nature. The theory that all types of matter have wave-like properties was first proposed by Louis de Broglie in 1924. Matter waves are discussed in Photons and Matter Waves .

Mechanical Waves

Mechanical waves exhibit characteristics common to all waves, such as amplitude, wavelength, period, frequency, and energy. All wave characteristics can be described by a small set of underlying principles.

The simplest mechanical waves repeat themselves for several cycles and are associated with simple harmonic motion. These simple harmonic waves can be modeled using some combination of sine and cosine functions. For example, consider the simplified surface water wave that moves across the surface of water as illustrated in Figure 16.3 . Unlike complex ocean waves, in surface water waves, the medium, in this case water, moves vertically, oscillating up and down, whereas the disturbance of the wave moves horizontally through the medium. In Figure 16.3 , the waves causes a seagull to move up and down in simple harmonic motion as the wave crests and troughs (peaks and valleys) pass under the bird. The crest is the highest point of the wave, and the trough is the lowest part of the wave. The time for one complete oscillation of the up-and-down motion is the wave’s period T . The wave’s frequency is the number of waves that pass through a point per unit time and is equal to f = 1 / T . f = 1 / T . The period can be expressed using any convenient unit of time but is usually measured in seconds; frequency is usually measured in hertz (Hz), where 1 Hz = 1 s −1 . 1 Hz = 1 s −1 .

The length of the wave is called the wavelength and is represented by the Greek letter lambda ( λ ) ( λ ) , which is measured in any convenient unit of length, such as a centimeter or meter. The wavelength can be measured between any two similar points along the medium that have the same height and the same slope. In Figure 16.3 , the wavelength is shown measured between two crests. As stated above, the period of the wave is equal to the time for one oscillation, but it is also equal to the time for one wavelength to pass through a point along the wave’s path.

The amplitude of the wave ( A ) is a measure of the maximum displacement of the medium from its equilibrium position. In the figure, the equilibrium position is indicated by the dotted line, which is the height of the water if there were no waves moving through it. In this case, the wave is symmetrical, the crest of the wave is a distance + A + A above the equilibrium position, and the trough is a distance − A − A below the equilibrium position. The units for the amplitude can be centimeters or meters, or any convenient unit of distance.

The water wave in the figure moves through the medium with a propagation velocity v → . v → . The magnitude of the wave velocity is the distance the wave travels in a given time, which is one wavelength in the time of one period, and the wave speed is the magnitude of wave velocity. In equation form, this is

This fundamental relationship holds for all types of waves. For water waves, v is the speed of a surface wave; for sound, v is the speed of sound; and for visible light, v is the speed of light.

Transverse and Longitudinal Waves

We have seen that a simple mechanical wave consists of a periodic disturbance that propagates from one place to another through a medium. In Figure 16.4 (a), the wave propagates in the horizontal direction, whereas the medium is disturbed in the vertical direction. Such a wave is called a transverse wave . In a transverse wave, the wave may propagate in any direction, but the disturbance of the medium is perpendicular to the direction of propagation. In contrast, in a longitudinal wave or compressional wave, the disturbance is parallel to the direction of propagation. Figure 16.4 (b) shows an example of a longitudinal wave. The size of the disturbance is its amplitude A and is completely independent of the speed of propagation v .

A simple graphical representation of a section of the spring shown in Figure 16.4 (b) is shown in Figure 16.5 . Figure 16.5 (a) shows the equilibrium position of the spring before any waves move down it. A point on the spring is marked with a blue dot. Figure 16.5 (b) through (g) show snapshots of the spring taken one-quarter of a period apart, sometime after the end of` the spring is oscillated back and forth in the x -direction at a constant frequency. The disturbance of the wave is seen as the compressions and the expansions of the spring. Note that the blue dot oscillates around its equilibrium position a distance A , as the longitudinal wave moves in the positive x -direction with a constant speed. The distance A is the amplitude of the wave. The y -position of the dot does not change as the wave moves through the spring. The wavelength of the wave is measured in part (d). The wavelength depends on the speed of the wave and the frequency of the driving force.

Waves may be transverse, longitudinal, or a combination of the two. Examples of transverse waves are the waves on stringed instruments or surface waves on water, such as ripples moving on a pond. Sound waves in air and water are longitudinal. With sound waves, the disturbances are periodic variations in pressure that are transmitted in fluids. Fluids do not have appreciable shear strength, and for this reason, the sound waves in them are longitudinal waves. Sound in solids can have both longitudinal and transverse components, such as those in a seismic wave. Earthquakes generate seismic waves under Earth’s surface with both longitudinal and transverse components (called compressional or P-waves and shear or S-waves, respectively). The components of seismic waves have important individual characteristics—they propagate at different speeds, for example. Earthquakes also have surface waves that are similar to surface waves on water. Ocean waves also have both transverse and longitudinal components.

Example 16.1

Wave on a string.

• The speed of the wave can be derived by dividing the distance traveled by the time.
• The period of the wave is the inverse of the frequency of the driving force.
• The wavelength can be found from the speed and the period v = λ / T . v = λ / T .
• The first wave traveled 30.00 m in 6.00 s: v = 30.00 m 6.00 s = 5.00 m s . v = 30.00 m 6.00 s = 5.00 m s .
• The period is equal to the inverse of the frequency: T = 1 f = 1 2.00 s −1 = 0.50 s . T = 1 f = 1 2.00 s −1 = 0.50 s .
• The wavelength is equal to the velocity times the period: λ = v T = 5.00 m s ( 0.50 s ) = 2.50 m . λ = v T = 5.00 m s ( 0.50 s ) = 2.50 m .

Significance

Check your understanding 16.1.

When a guitar string is plucked, the guitar string oscillates as a result of waves moving through the string. The vibrations of the string cause the air molecules to oscillate, forming sound waves. The frequency of the sound waves is equal to the frequency of the vibrating string. Is the wavelength of the sound wave always equal to the wavelength of the waves on the string?

Example 16.2

Characteristics of a wave.

• The amplitude and wavelength can be determined from the graph.
• Since the velocity is constant, the velocity of the wave can be found by dividing the distance traveled by the wave by the time it took the wave to travel the distance.
• The period can be found from v = λ T v = λ T and the frequency from f = 1 T . f = 1 T .
• The distance the wave traveled from time t = 0.00 s t = 0.00 s to time t = 3.00 s t = 3.00 s can be seen in the graph. Consider the red arrow, which shows the distance the crest has moved in 3 s. The distance is 8.00 cm − 2.00 cm = 6.00 cm . 8.00 cm − 2.00 cm = 6.00 cm . The velocity is v = Δ x Δ t = 8.00 cm − 2.00 cm 3.00 s − 0.00 s = 2.00 cm/s . v = Δ x Δ t = 8.00 cm − 2.00 cm 3.00 s − 0.00 s = 2.00 cm/s .
• The period is T = λ v = 8.00 cm 2.00 cm/s = 4.00 s T = λ v = 8.00 cm 2.00 cm/s = 4.00 s and the frequency is f = 1 T = 1 4.00 s = 0.25 Hz . f = 1 T = 1 4.00 s = 0.25 Hz .

Check Your Understanding 16.2

The propagation velocity of a transverse or longitudinal mechanical wave may be constant as the wave disturbance moves through the medium. Consider a transverse mechanical wave: Is the velocity of the medium also constant?

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23.2: Electromagnetic Waves and their Properties

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learning objectives

• Explain the meaning and importance of Maxwell’s equations

Maxwell’s Equations

Maxwell’s equations are a set of four partial differential equations that, along with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits.

Named after esteemed physicist James Clerk Maxwell, the equations describe the creation and propagation of electric and magnetic fields. Fundamentally, they describe how electric charges and currents create electric and magnetic fields, and how they affect each other.

Maxwell’s equations can be divided into two major subsets. The first two, Gauss’s law and Gauss’s law for magnetism, describe how fields emanate from charges and magnets respectively. The other two, Faraday’s law and Ampere’s law with Maxwell’s correction, describe how induced electric and magnetic fields circulate around their respective sources.

Each of Maxwell’s equations can be looked at from the “microscopic” perspective, which deals with total charge and total current, and the “macroscopic” set, which defines two new auxiliary fields that allow one to perform calculations without knowing microscopic data like atomic-level charges.

Gauss’s Law

Gauss’s law relates an electric field to the charge(s) that create(s) it. The field (E) points towards negative charges and away from positive charges, and from the microscopic perspective, is related to charge density (ρ) and vaccuum permittivity (ε 0 , or permittivity of free space) as:

\[\nabla \cdot \mathbf { E } = \dfrac { \rho } { \epsilon _ { 0 } }\]

Gauss’s Law basically says that a net amount of charge contained within a region of space will generate an electric field that emanates through the surface that surrounds that region.

Example of Gauss’s Law : A positive charge contained within a region of space creates an electric field that emanates from the surface of that region.

Gauss’s Law for Magnetism

Gauss’s law for magnetism states that there are no “magnetic charges (or monopoles)” analogous to electric charges, and that magnetic fields are instead generated by magnetic dipoles . Such dipoles can be represented as loops of current, but in many ways are similar in appearance to positive and negative “magnetic charges” that are inseparable and thus have no formal net “magnetic charge.”

Magnetic field lines form loops such that all field lines that go into an object leave it at some point. Thus, the total magnetic flux through a surface surrounding a magnetic dipole is always zero.

Field lines caused by a magnetic dipole : The field lines created by this magnetic dipole either form loops or extend infinitely.

The differential form of Gauss’s law for magnetic for magnetism is

\[\nabla \cdot \mathbf { B } = \mathbf { 0 }\]

Faraday’s Law

Faraday’s law describes how a time-varying magnetic field (or flux) induces an electric field. The principle behind this phenomenon is used in many electric generators. Both macroscopic and microscopic differential equations are the same, relating electric field (E) to the time-partial derivative of magnetic field (B):

\[\nabla \times \mathbf { E } = - \frac { \partial \mathbf { B } } { \partial \mathbf { t } }\]

Ampere’s Circuital Law (with Maxwell’s correction)

Ampere’s law originally stated that magnetic field could be created by electrical current. Maxwell added a second source of magnetic fields in his correction: a changing electric field (or flux), which would induce a magnetic field even in the absence of an electrical current. He named the changing electric field “displacement current.”

Maxwell’s correction shows that self-sustaining electromagnetic waves (light) can travel through empty space even in the absence of moving charges or currents, with the electric field component and magnetic field component each continually changing and each perpetuating the other.

Electromagnetic Waves : Electric (red) and magnetic (blue) waves propagate in phase sinusoidally, and perpendicularly to one another.

The microscopic approach to the Maxwell-corrected Ampere’s law relates magnetic field (B) to current density (J, or current per unit cross sectional area) and the time-partial derivative of electric field (E):

\[\nabla \times \mathbf { B } = \mu _ { 0 } \mathbf { J } + \mu _ { 0 } \epsilon _ { 0 } \frac { \partial \mathbf { E } } { \partial t }\]

The Production of Electromagnetic Waves

Electromagnetic waves are the combination of electric and magnetic field waves produced by moving charges.

• Explain the self-perpetuating behavior of an electromagnetic wave

Electromagnetic waves

Electromagnetic radiation, is a form of energy emitted by moving charged particles. As it travels through space it behaves like a wave, and has an oscillating electric field component and an oscillating magnetic field. These waves oscillate perpendicularly to and in phase with one another.

Electromagnetic Wave : Electromagnetic waves are a self-propagating transverse wave of oscillating electric and magnetic fields. The direction of the electric field is indicated in blue, the magnetic field in red, and the wave propagates in the positive x-direction. Notice that the electric and magnetic field waves are in phase.

The creation of all electromagnetic waves begins with a charged particle. This charged particle creates an electric field (which can exert a force on other nearby charged particles). When it accelerates as part of an oscillatory motion, the charged particle creates ripples, or oscillations, in its electric field, and also produces a magnetic field (as predicted by Maxwell’s equations).

Once in motion, the electric and magnetic fields created by a charged particle are self-perpetuating—time-dependent changes in one field (electric or magnetic) produce the other. This means that an electric field that oscillates as a function of time will produce a magnetic field, and a magnetic field that changes as a function of time will produce an electric field. Both electric and magnetic fields in an electromagnetic wave will fluctuate in time, one causing the other to change.

Electromagnetic waves are ubiquitous in nature (i.e., light) and used in modern technology—AM and FM radio, cordless and cellular phones, garage door openers, wireless networks, radar, microwave ovens, etc. These and many more such devices use electromagnetic waves to transmit data and signals.

All the above sources of electromagnetic waves use the simple principle of moving charge, which can be easily modeled. Placing a coin in contact with both terminals of a 9-volt battery produces electromagnetic waves that can be detected by bringing the antenna of a radio (tuned to a static-producing station) within a few inches of the point of contact.

Energy and Momentum

Electromagnetic waves have energy and momentum that are both associated with their wavelength and frequency.

• Relate energy of an electromagnetic wave with the frequency and wavelength

Electromagnetic radiation can essentially be described as photon streams. These photons are strictly defined as massless, but have both energy and surprisingly, given their lack of mass, momentum, which can be calculated from their wave properties.

Waves were poorly understood until the 1900s, when Max Planck and Albert Einstein developed modern corrections to classical theory.

Planck theorized that “black bodies” (thermal radiators) and other forms of electromagnetic radiation existed not as spectra, but in discrete, “quantized” form. In other words, there were only certain energies an electromagnetic wave could have. In his work he developed what is now known as “Planck’s constant,” which is approximately equal to 6.626×10 -34 J·s.

The energy (E) of a photon can be related to its frequency (f) by Planck’s constant (h):

\[\mathrm { E } = \mathrm { hf } = \frac { \mathrm { hc } } { \lambda }\]

The ratio of speed of light (c) to wavelength (λ) can be substituted in place of f to give the same equation to energy in different terms. Note that energy cannot take any value: it can only exist in increments of frequency times Planck’s constant (or Planck’s constant times c divided by wavelength). Energy of a wave is therefore “quantized. ”

Wavelength : Wavelength of the sinusoidal function is represented by λ.

Momentum is classically defined as the product of mass and velocity and thus would intuitively seem irrelevant to a discussion of electromagnetic radiation, which is both massless and composed of waves.

However, Einstein proved that light can act as particles in some circumstances, and that a wave-particle duality exists. And, given that he related energy and mass (E=mc 2 ), it becomes more conceivable that a wave (which has an energy value) not only has an equation to mass but a momentum as well.

And indeed, Einstein proved that the momentum (p) of a photon is the ratio of its energy to the speed of light.

\[\mathrm { p } = \dfrac { \mathrm { E } } { \mathrm { c } } = \dfrac { \mathrm { hf } } { \mathrm { c } } = \dfrac { \mathrm { h } } { \lambda }\]

Substituting E with hc/λ cancels the c terms, making momentum also equal to the simple ratio of Planck’s constant to wavelength.

The Speed of Light

The speed of light in a vacuum is one of the most fundamental constant in physics, playing a pivotal role in modern physics.

• Relate speed of light with the index of refraction of the medium

The speed of light is generally a point of comparison to express that something is fast. shows a scale representation of the time it takes a beam of light to reach the moon from Earth. But what exactly is the speed of light?

Light Going from Earth to the Moon : A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.

It is just that: the speed of a photon or light particle. The speed of light in a vacuum (commonly written as c) is 299,792,458 meters per second. This is a universal physical constant used in many areas of physics. For example, you might be familiar with the equation:

\[\mathrm { E } = \mathrm { mc } ^ { 2 }\]

where E = Energy and m = mass. This is known as the mass-energy equivalence, and it uses the speed of light to interrelate space and time. This not only explains the energy a body of mass contains, but also explains the hindrance mass has on speed.

There are many uses for the speed of light in a vacuum, such as in special relativity, which says that c is the natural speed limit and nothing can move faster than it. However, we know from our understanding of physics (and previous atoms) that the speed at which something travels also depends on the medium through which it is traveling. The speed at which light propagates through transparent materials (air, glass, etc.,) is dependent on the refractive index of that material, n:

\[\mathrm { v } = \dfrac { \mathrm { c } } { \mathrm { n } }\]

where v = actual velocity of light moving through the medium, c = speed of light in a vacuum, and n = refractive index of medium. The refractive index of air is about 1.0003, and from this equation we can find that the speed of visible light in air is about 90 km/s slower than c.

As mentioned earlier, the speed of light (usually of light in a vacuum) is used in many areas of physics. Below is an example of an application of the constant c.

The Lorentz Factor

Fast-moving objects exhibit some properties that are counterintuitive from the perspective of classical mechanics. For example, length contracts and time dilates (runs slower) for objects in motion. The effects are typically minute, but are noticeable at sufficiently high speeds. The Lorentz factor (γ) is the factor by which length shortens and time dilates as a function of velocity (v):

\[\gamma = \left( 1 - \mathrm{ v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 } \gamma = \left( 1 - \mathrm { v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 } \gamma = \left( 1 - \mathrm { v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 }\]

At low velocities, the quotient of v 2 /c 2 is sufficiently close to 0 such that γ is approximately 1. However, as velocity approaches c, γ increases rapidly towards infinity.

The Doppler Effect

The Doppler Effect is the change in a wave’s perceived frequency that results from the source’s motion, the observer, and the medium.

• Give examples of daily observations of the Doppler effect

The Doppler effect is a periodic event’s change in frequency for an observer in motion relative to the event’s source. Typically, this periodic event is a wave.

Most people have experienced the Doppler effect in action. Consider an emergency vehicle in motion, sounding its siren. As it approaches an observer, the pitch of the sound (its frequency) sounds higher than it actually is. When the vehicle reaches the observer, the pitch is perceived as it actually is. When the vehicle continues away from the observer, the pitch is perceived as lower than it actually is. From the perspective of an observer inside the vehicle, the pitch of the siren is constant.

The Doppler Effect and Sirens : Waves emitted by a siren in a moving vehicle

The difference in the perceived pitch depending on observer location can be explained by the fact that the siren’s position changes as it emits waves. A wave of sound is emitted by a moving vehicle every millisecond. The vehicle ‘chases’ each wave in one direction. By the time the next wave is emitted, it is closer (relative to an onlooker ahead of the vehicle) to the previous wave than the wave’s frequency would suggest. Relative to an onlooker behind the vehicle, the second wave is further from the first wave than one would expect, which suggests a lower frequency.

The Doppler effect can be caused by any kind of motion. In the example above, the siren moved relative to a stationary observer. If the observer moves relative to the stationary siren, the observer will notice the Doppler effect on the pitch of the siren. Finally, if the medium through which the waves propagate moves, the Doppler effect will be noticed even for a stationary observer. An example of this phenomenon is wind.

Quantitatively, the Doppler effect can be characterized by relating the frequency perceived (f) to the velocity of waves in the medium (c), the velocity of the receiver relative to the medium (v r ), the velocity of the source relative to the medium (v s ), and the actual emitted frequency (f 0 ):

\[\mathrm { f } = \left( \dfrac { \mathrm { c } + \mathrm { v } _ { \mathrm { r } } } { \mathrm { c } + \mathrm { v } _ { \mathrm { s } } } \right) \mathrm { f } _ { 0 }\]

The Doppler Effect : Wavelength change due to the motion of source

Momentum Transfer and Radiation Pressure Atom

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation.

• Explain formation of radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation. EM radiation (or photon, which is a quantum of light) carries momentum; this momentum is transferred to an object when the radiation is absorbed or reflected. Perhaps one of the most well know examples of the radiation pressure would be comet tails. Haley’s comet is shown in.

Halley’s Comet : As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus. The streams of dust and gas thus released form an atmosphere around the comet (called the coma), and the force exerted on the coma by the Sun’s radiation pressure and solar wind cause the formation of an enormous tail that points away from the Sun.

Although radiation pressure can be understood using classical electrodynamics, here we will examine the quantum mechanical argument. From the perspective of quantum theory, light is made of photons: particles with zero mass but which carry energy and – importantly in this argument – momentum. According to special relativity, because photons are devoid of mass, their energy (E) and momentum (p) are related by E=pc.

Now consider a beam of light perpendicularly incident on a surface, and let us assume the beam of light is totally absorbed. The momentum the photons carry is a conserved quantity (i.e., it cannot be destroyed) so it must be transferred to the surface; thus the absorption of the light beam causes the surface to gain momentum. Newton’s Second Law tells us that force equals rate of change of momentum; thus during each second, the surface experiences a force (or pressure, as pressure is force per unit area) due to the momentum the photons transfer to it.

This gives us: pressure = momentum transferred per second per unit area = energy deposited per second per unit area / c = I/c, (where I is the intensity of the beam of light).

Laser Cooling

There are many variations of laser cooling, but they all use radiation pressure to remove energy from atomic gases (and therefore cool the sample). In laser cooling (sometimes called Doppler cooling), the frequency of light is tuned slightly below an electronic transition in the atom. Because light is detuned to the “red” (i.e., at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion (typical setups applies three opposing pairs of laser beams as in ).

The Magneto Optical Trap : Experimental setup of Magneto Optical Trap (MOT), which uses radiation pressure to cool atomic species. Atoms are slowed down by absorbing (and emitting) photons.

In each scattering event, the atom loses a momentum equal to the momentum of the photon. If the atom (which is now in the excited state) then emits a photon spontaneously, it will be kicked by the same amount of momentum, only in a random direction. Since the initial momentum loss was opposite to the direction of motion (while the subsequent momentum gain was in a random direction), the overall result of the absorption and emission process is to reduce the speed of the atom. If the absorption and emission are repeated many times, the average speed (and therefore the kinetic energy ) of the atom will be reduced. Since the temperature of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms. Simple laser cooling setups can produce a cold sample of atomic gases at around 1mK (=10 -3 K) starting from a room temperature gas.

• Maxwell’s four equations describe how electric charges and currents create electric and magnetic fields, and how they affect each other.
• Gauss’s law relates an electric field to the charge(s) that create(s) it.
• Gauss’s law for magnetism states that there are no “magnetic charges” analogous to electric charges, and that magnetic fields are instead generated by magnetic dipoles.
• Faraday’s law describes how a time-varying magnetic field (or flux ) induces an electric field. The principle behind this phenomenon is used in many electric generators.
• Ampere ‘s law originally stated that a magnetic field is created by an electrical current. Maxwell added that a changing electric flux can also generate a magnetic field.
• Electromagnetic waves consist of both electric and magnetic field waves. These waves oscillate in perpendicular planes with respect to each other, and are in phase.
• The creation of all electromagnetic waves begins with an oscillating charged particle, which creates oscillating electric and magnetic fields.
• Once in motion, the electric and magnetic fields that a charged particle creates are self-perpetuating: time-dependent changes in one field (electric or magnetic) produce the other.
• Max Planck proved that energy of a photon (a stream of which is an electromagnetic wave ) is quantized and can exist in multiples of “Planck’s constant” (denoted as h, approximately equal to 6.626×10 -34 J·s).
• \(\mathrm { E } = \mathrm { hf } = \frac { \mathrm { hc } } { \lambda } \)describes the energy (E) of a photon as a function of frequency (f), or wavelength (λ).
• \(\mathrm { p } = \frac { \mathrm { E } } { \mathrm { c } } = \frac { \mathrm { hf } } { \mathrm { c } } = \frac { \mathrm { h } } { \lambda }\) describes the momentum (p) of a photon as a function of its energy, frequency, or wavelength.
• The maximum possible value for the speed of light is that of light in a vacuum, and this speed is used for a constant in many area of physics.
• c is the symbol used to represent the speed of light in a vacuum, and its value is 299,792,458 meters per second.
• When light travels through medium, its speed is hindered by the index of refraction of that medium. Its actual speed can be found with: \(v=\frac{c}{n}\).
• The Doppler effect is very commonly observed in action.
• The Doppler effect can be observed in the apparent change in pitch of a siren on an emergency vehicle, according to a stationary observer.
• The observer will notice the Doppler effect on the pitch of the stationary siren when moving relative to its pitch, or if the medium moves when the observer is stationary.
• Photons carry momentum (p = E/c). When photons are absorbed or reflected on a surface, the surface receives momentum kicks. This momentum transfer leads to radiation pressure.
• Electromagnetic radiation applies radiation pressure equal to the Intensity (of light beam) divided by c (speed of light).
• Laser cooling uses radiation pressure to remove energy from atomic gases. The technique can produce cold samples of gases at 1mK or so.
• differential equation : An equation involving the derivatives of a function.
• flux : A quantitative description of the transfer of a given vector quantity through a surface. In this context, we refer to the electric flux and magnetic flux.
• electromagnetic wave : A wave of oscillating electric and magnetic fields.
• phase : Waves are said to be “in phase” when they begin at the same part (e.g., crest) of their respective cycles.
• photon : The quantum of light and other electromagnetic energy, regarded as a discrete particle having zero rest mass, no electric charge, and an indefinitely long lifetime.
• wavelength : The length of a single cycle of a wave, as measured by the distance between one peak or trough of a wave and the next; it is often designated in physics as λ, and corresponds to the velocity of the wave divided by its frequency.
• frequency : The quotient of the number of times n a periodic phenomenon occurs over the time t in which it occurs: f = n / t.
• special relativity : A theory that (neglecting the effects of gravity) reconciles the principle of relativity with the observation that the speed of light is constant in all frames of reference.
• refractive index : The ratio of the speed of light in air or vacuum to that in another medium.
• doppler effect : Apparent change in frequency of a wave when the observer and the source of the wave move relative to each other.
• classical electrodynamics : A branch of theoretical physics that studies consequences of the electromagnetic forces between electric charges and currents.

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It’s a wonderful world — and universe — out there.

Come explore with us!  

Science News Explores

Understanding light and other forms of energy on the move.

This radiation includes visible light, radio signals — even medical X-rays

Light is a form of energy created by the movement of electrons. Different wavelengths appear as different colors, although most wavelengths are not visible to the human eye.

Natasha Hartano/Flickr ( CC BY-NC 2.0 ); adapted by L. Steenblik Hwang

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• Google Classroom

By Jennifer Look

July 16, 2020 at 6:30 am

Light is a form of energy that travels as waves. Their length — or wavelength — determines many of light’s properties. For instance, wavelength accounts for light’s color and how it will interact with matter. The range of wavelengths, from super short to very, very long, is known as the light spectrum. Whatever its wavelength, light will radiate out infinitely unless or until it is stopped. As such, light is known as radiation.

Light’s formal name is electromagnetic radiation. All light shares three properties. It can travel through a vacuum. It always moves at a constant speed, known as the speed of light, which is 300,000,000 meters (186,000 miles) per second in a vacuum. And the wavelength defines the type or color of light.

Just to make things interesting, light also can behave as photons , or particles. When looked at this way, quantities of light can be counted, like beads on a string.

Humans have evolved to sense a small part of the light spectrum. We know these wavelengths as “visible” light. Our eyes contain cells known as rods and cones. Pigments in those cells can interact with certain wavelengths (or photons) of light. When this happens, they create signals that travel to the brain. The brain interprets the signals from different wavelengths (or photons) as different colors.

The longest visible wavelengths are around 700 nanometers and appear red. The range of visible light ends around 400 nanometers. Those wavelengths appear violet. The whole rainbow of colors falls in between.

Most of the light spectrum, however, falls outside that range. Bees, dogs and even a few people can see ultraviolet (UV) light . These are wavelengths a bit shorter than violet ones. Even those of us without UV vision can still respond to UV light, however. Our skin will redden or even burn when it encounters too much.

Many things emit heat in the form of infrared light. As that name suggests, infrared wavelengths are somewhat longer than red’s. Mosquitoes and pythons can see in this range. Night-vision goggles work by detecting infrared light.

Light also comes in many other types. Light with really short, high-energy waves can be gamma rays and X-rays (used in medicine). Long, low-energy waves of light fall in the radio and microwave part of the spectrum.

Desiré Whitmore is a physics educator at the Exploratorium in San Francisco, Calif. Teaching people about light as radiation can be difficult, she says. “People are afraid of the word ‘radiation.’ But all it means is that something is moving outward.”

The sun emits lots of radiation in wavelengths that span from X-rays to infrared. Sunlight provides almost all of the energy required for life on Earth. Small, cool objects release much less radiation. But every object emits some. That includes people. We give off small amounts of infrared light generally referred to as heat.

Whitmore points to her cell phone as a common source of many types of light. Smartphones use visible wavelengths to light up the screen display. Your phone talks to other phones via radio waves. And the camera has the ability to detect infrared light that human eyes cannot see. With the right app, the phone transforms this infrared light into visible light that we can see on the phone’s screen.

“This is fun to try out with your cell phone’s front-facing camera,” Whitmore says. Use a remote control for a television or other device. Its light is infrared, she notes, “so we cannot see it. But when you point the controller at your phone’s camera and press a button, “you can see a bright pink light appear on the screen!”   

“All these different types of radiation help improve our lives,” Whitmore says. They “have been shown to be safe when used in reasonable amounts,” she notes — but can be “dangerous when you use too much of it.”

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Does Light Travel Forever?

Most recent answer: 01/23/2013

Hi Raja, Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears. In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through). There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us! David Schmid

(published on 01/23/2013)

Follow-Up #1: How far does light go?

Light just keeps going and going until it bumps into something.  Then it can either be reflected or absorbed.  Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.   

Light has some interesting properties.   It comes in lumps called photons.  These photons carry energy and momentum in specific amounts related to the color of the light.  There is much to learned about light.   I suggest you log in to our website and type  LIGHT into the search box.   Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" ,  the answer is yes.

(published on 12/03/2015)

Follow-Up #2: less than one photon?

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

(published on 02/04/2016)

Follow-Up #3: stars too far away to see?

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

(published on 07/22/2016)

Follow-Up #4: light going out to space

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

(published on 09/01/2016)

Follow-Up #5: end of the universe?

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

(published on 03/26/2017)

Follow-Up #6: seeing black holes

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

(published on 01/29/2018)

Follow-up on this answer

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Middle school physics - NGSS

Course: middle school physics - ngss   >   unit 4, mechanical waves and light.

• Understand: mechanical waves and light

Key points:

• Sound waves, water waves, and seismic waves are all types of mechanical waves.
• Light is a form of electromagnetic wave.
• A light wave’s amplitude determines how intense, or bright, it is. Its frequency determines the light wave’s color.
• A sound wave’s amplitude determines how loud it is. Its frequency determines the sound wave’s pitch.

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Electromagnetic Radiation

Claimed by Zoila de Leon (Fall 2023)

• 1.1 What is a Electromagnetic(EM) Radiation?
• 1.2 General Properties
• 1.3 Problem Solving Method and Equations
• 1.4 Fields Made by Charges and Fields Made by Monopoles
• 2 The EM Spectrum
• 3 Waves and Fields
• 4 A Mathematical Model
• 5 Connectedness: X-Rays
• 6.1 Practice Problems (new section by Zoila)
• 7 References

What is a Electromagnetic(EM) Radiation?

Electromagnetic radiation is a form of energy that is all around us and takes many forms, such as radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Before 1873, electricity and magnetism were thought to be two different forces. However, in 1873, Scottish Physicist James Maxwell developed his famous theory of electromagnetism. There are four main electro magnetic interactions according to Maxwell:

• The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them
• Magnetic poles come in pairs that attract and repel each other much as electric charges do
• An electric current in a wire produces a magnetic field whose direction depends on the direction of the current
• A moving electric field produces a magnetic field, and vice versa

General Properties

The four Maxwell's Equations provide a complete description of possible spatial patterns of electric and magnetic field in space.

• The Ampere-Maxwell Law
• Gauss's Law
• Faraday's Law

Other than Maxwell's Four equations, there are general properties of all electromagnetic radiation:

• Electromagnetic radiation can travel through empty space. Most other types of waves must travel through some sort of substance. For example, sound waves need either a gas, solid, or liquid to pass through in order to be heard
• The speed of light is always a constant (3 x 10^8 m/s)
• Wavelengths are measured between the distances of either crests or troughs. It is usually characterized by the Greek symbol λ (lambda).

Electromagnetic waves are the self-propagating, mutual oscillation of electric and magnetic fields. The propagation of electromagnetic energy is often referred to as radiation. We can also say that the 'pulse' of these moving fields result in radiation (7).

The equation for propagation is E=cB with c being the speed of light. This equation is derived from combining the two equations E=vB and B=u0e0vE, proving that v is equal to 3e8 meters/second.

Problem Solving Method and Equations

To go about solving/analyzing mathematically an electromagnetic field using Maxwell's equations,this is how we proceed (7)

• Establish the space and time in which the electric and magnetic fields are present
• Check that Maxwell's equations can be applied in the situation above
• Check when the charge accelerates, it produces these fields and therefore radiation
• Show how these fields would interact with matter

The equation of the Radiative Electric Field is: E= 1/(4πe0)*-qa/(c^2r) where a is the acceleration of the particle, c is the speed of light and r is the distance from the original location of the charge to right before the kink. This kink happens on the electric field because of the slight delay when the charge is moved.

Fields Made by Charges and Fields Made by Monopoles

We can differentiate fields made by charges and the ones made by magnetic monopoles. (7) For fields made by charges, when the charge is

• at rest, E=1/r^2 and B=0
• constant speed, E=1/r^2 and B=1/r^2
• accelerating, E=1/r and B=1/r

For fields made by magnetic monopoles, the first point would have E and B switched.

The EM Spectrum

EM spectrum is a span of enormous range of wavelengths and frequencies. The EM spectrum is generally divided into 7 different regions, in order of decreasing wavelength and increasing energy and frequency. It ranges from Gamma rays to Long Radio Waves. Following are the lists of waves:

• Visible Light
• Infrared Rays
• Long radio waves

Although all these waves do different things, there is one thing in common : They all travel in waves.

Infrared radiation can be released as heat or thermal energy. It can also be bounced back, which is called near infrared because of its similarities with visible light energy. Infrared Radiation is most commonly used in remote sensing as infrared sensors collect thermal energy, providing us with weather conditions.

Visible Light is the only part of the electromagnetic spectrum that humans can see with a naked eye. This part of the spectrum includes a range of different colors that all represent a particular wavelength. Rainbows are formed in this way; light passes through matter in which it is absorbed or reflected based on its wavelength. As a result, some colors are reflected more than other, leading to the creation of a rainbow.

Waves and Fields

As we learned in class, electric field is produced when an electron is accelerating. Likewise, EM radiation is created when an atomic particle, like an electron, is accelerated by an electric field. The movement like this produces oscillating electric and magnetic fields, which travel at right angles to each other in a bundle of light energy called a photon. Photons travel in a harmonic wave at the fastest speed possible in the universe.

Electromagnetic waves are formed when an electric field couples with a magnetic field. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

A wavelength (in m) is the distance between two consecutive peaks of a wave. Frequency is the number of waves that form in a given length of time. A wavelength and frequency are interrelated. A short wavelength indicates that the frequency will be higher because one cycle can pass in a shorter amount of time. Likewise, a longer wavelength has a lower frequency because each cycle takes longer to complete.

Waves can be classified according to their nature:

• Mechanical waves
• Electromagnetic waves

Mechanical Waves

Mechanical waves require a medium (matter) to travel through. Examples are sound waves, water waves, ripples in strings or springs.

Water Waves

Sound Waves

Electromagnetic Waves

Electromagnetic waves do not require a medium (matter) to travel through - they can travel through space. Examples are radio waves, visible light, x-rays.

Radio Waves

Visible Lights

A Mathematical Model

The position of the particle is defined by a sine wave:

y = ymaxsin(wt)

Where w is the angular frequency

Amplitude is the distance from the maximum vertical displacement of the wave to the middle of the wave. The Amplitude of the sinusoidal Wave is the height of the peak in the wave measured from the zero line. This measures the magnitude of oscillation of a particular wave. The Amplitude is important because it tells you the intensity or brightness of a wave in comparison with other waves.

The period of the wave is the time between crests in seconds(s).

T = 2pi/w-----(units of seconds)

Frequency is the number of cycles per second, and is expressed as sec-1 or Hertz(Hz). Frequency is directly proportional to energy and can be express as "

E = hv where E is energy, h is Planck's constant ( 6.62607*10^-34J) and v is frequency

f = 1/T f = w/2pi----(Units Hertz)

Wavelength is the distance between crests in meters. Wavelength is equal to the speed of light times frequency. Longer wavelength waves such as radio waves carry low energy; this is why we can listen to the radio without any harmful consequences. Shorter wavelength waves such as x-rays carry higher energy that can be hazardous to our health.

Wavelength and Frequency

The speed of light is the multiplication of the wavelength and frequency.

This diagram shows all properties of waves:

ENERGY FLUX

Is defined by the following equation:

Connectedness: X-Rays

Electromagnetic Radiation while commonly thought of as only including visible light, radio waves, UV waves, and gamma rays; also include X-rays. In 1895, X-rays were initially discovered by William Roentgen, who accidentally fell upon the most important discovery about his life (Figure 1). Roentgen was already working on cathode rays, and because of a fluorescent glow that occurred during his experiments, covered his experimental apparatus with heavy black paper. However, when he did this, he discovered a glow coming from a screen several feet away. Through many more experiments, he discovered that a new type of energy, not cathode rays, were the cause of the glow. He named them “x-rays” and received the 1901 Nobel Prize in Physics. Roentgen never patented his monumental discovery and as a result, numerous researchers set out to find a multitude of uses and capitalize on his work.

Primarily, people could now view objects that were hidden from plain view (i.e. scanners in airports). While X-rays are now used in 100’s of professions (security, chemistry, art galleries), its most important function is to view bones to determine abnormalities in humans. In fact, one of Roentgen’s first x-rays was of his wife’s hand (Figure 2). X-rays fall under the scope of electromagnetic radiation because, like all E.R. waves, it is comprised of photons. X-rays have wavelengths between 0.01 to 10 nanometers and fall between UV and Gamma Waves on the E.R. spectrum (Figure 3). There are two main methods in which an x-ray may be formed. Both require a vacuum-filled tube called an x-ray tube (Figure 4). With an anode on one end and a cathode on the other, an electric current is applied and a high energy electron is projected from the cathode, through the vacuum, and at the anode. In the characteristic x-ray generation approach, the electron from the cathode collides with an inner shell electron on an atom on the anode (Figure 5). Both of these electrons are ejected from the atom and an outer shell electron takes the place of the inner shell one. Because the outer electron must have a lower energy to fill the inner shell hole, it releases a photon with the equivalent energy of the difference between the two energy levels in the atom. This photon is the x-ray that is used to view objects such as bones.

In the Bremsstrahlung x-ray generation method, the electron from the cathode is slowed as it passes the nucleus of an atom at the anode (Figure 6). As it slows and its path is changed, the loses energy (kinetic energy). This energy is also released as a photon which is subsequently called an x-ray. Depending on the voltage and current of the tube and the material of the anode, different types (as in wavelengths and energy) of x-rays can be produced and each one. However, all X-rays will continue to pass through objects until it reaches a material dense that stops it. However, density of the material required depends on the energy of the x-ray. For example, during a medical x-ray, x-rays of a certain energy will pass through soft tissue (skin, organs, etc) but not through bones. The x-rays that pass through the soft tissue will strike the screen and the absence of the x-rays absorbed by the bones will cause a negative space on the screen. The areas where x-rays do not strike will form the image of the bone. While the principles remain the same, x-ray machines today use incredible sophisticated technology to specify the type of x-ray they want and have greatly increased in accuracy since Roentgen’s initial discovery.

• Information and photographs are pulled from references 1 through 5 cited below*

Already, during the Ancient Greek and Roman times, light was studied as the presence of deflection and refraction were noticed. Electromagnetic radiation of wavelengths in the early 19th century. The discovery of infrared radiation is ascribed to astronomer William Herschel, who published his results in 1800 before the Royal Society of London. Herschel used a glass Triangular prism (optics)|prism to refract light from the Sun and detected invisible rays that caused heating beyond the red part of the spectrum, through an increase in the temperature recorded with a thermometer. These "calorific rays" were later termed infrared.

In 1801, Rohann Ritter, discovered the presence of ultraviolet light using salts. It was known that light could darken some silver halides and while doing so, he realized that the region beyond the violet bar (therefore ultraviolet) was more effective in changing the color of the halides. However,in 1864, while summarizing the theories of his time accumulating into his famous set of Maxwell equations, James Clerk Maxwell managed to deduce the speed of light being around 3e8 meters per second. This was instrumental in creating the rest of the spectrum.

In 1887-1888 Physicist Heinrich Hertz not only tried to measure the velocity and frequency of electromagnetic radiation waves at other parts of the known spectrum of the time, but he was also able to prove that Maxwell's findings were correct. He did this on the microwave radiation as well.

The discovery of X-rays occurred in 1895 by Wilhelm Rontgen when his barium platinocyanide detector screen began to glow under the presence of a discharge that passed through a cathode ray tube although the latter was completely covered. Once he determined its possible use, he tried to look at his wife's hand using this new discovery. However x-ray spectroscopy was not institutionalized until later by Karl Manne Siegbahn.

In 1900, Paul Villard discovered Gamma rays although he initially thought that they were particles similar to alpha and beta particles which were emitted during radiation. These 'particles' were later proven to be part of the electromagnetic spectrum.

Practice Problems (new section by Zoila)

1. Elert, Glenn. "X-rays." X-rays – The Physics Hypertextbook. N.p., n.d. Web. 08 Apr. 2017. http://physics.info/x-ray/

2."X-rays." X-rays. N.p., n.d. Web. 08 Apr. 2017. http://www.physics.isu.edu/radinf/xray.htm

3. "Basics of X-ray PhysicsX-ray production." Welcome to Radiology Masterclass. N.p., n.d. Web. 08 Apr. 2017. http://www.radiologymasterclass.co.uk/tutorials/physics/x-ray_physics_production#top_2nd_img

4. "X-Rays." Image: Electromagnetic Spectrum. N.p., n.d. Web. 08 Apr. 2017. https://www.boundless.com/physics/textbooks/boundless-physics-textbook/electromagnetic-waves-23/the-electromagnetic-spectrum-165/x-rays-597-11175/images/electromagnetic-spectrum/

5. "This Month in Physics History." American Physical Society. N.p., n.d. Web. 08 Apr. 2017. https://www.aps.org/publications/apsnews/200111/history.cfm

6. Editors, Spectroscopy. “The Electromagnetic Spectrum: A History.” Spectroscopy Home, 27 Oct. 2017, www.spectroscopyonline.com/electromagnetic-spectrum-history?id=&sk=&date=&&pageID=4.

7. Chabay, Ruth W., and Bruce A. Sherwood. Matter & Interaction II: Electric & Magnetic Interactions, Version 1.2. John Wiley & Sons, 2003.

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Katy Mersmann

1) electromagnetic fields, 2) magnetic explosions, 3) wave-particle interactions.

One hundred years ago today, on May 29, 1919, measurements of a solar eclipse offered verification for Einstein’s theory of general relativity. Even before that, Einstein had developed the theory of special relativity, which revolutionized the way we understand light. To this day, it provides guidance on understanding how particles move through space — a key area of research to keep spacecraft and astronauts safe from radiation.

The theory of special relativity showed that particles of light, photons, travel through a vacuum at a constant pace of 670,616,629 miles per hour — a speed that’s immensely difficult to achieve and impossible to surpass in that environment. Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light.

One of NASA’s jobs is to better understand how these particles are accelerated. Studying these superfast, or relativistic, particles can ultimately help protect missions exploring the solar system, traveling to the Moon, and they can teach us more about our galactic neighborhood: A well-aimed near-light-speed particle can trip onboard electronics and too many at once could have negative radiation effects on space-faring astronauts as they travel to the Moon — or beyond.

Here are three ways that acceleration happens.

Most of the processes that accelerate particles to relativistic speeds work with electromagnetic fields — the same force that keeps magnets on your fridge. The two components, electric and magnetic fields, like two sides of the same coin, work together to whisk particles at relativistic speeds throughout the universe.

In essence, electromagnetic fields accelerate charged particles because the particles feel a force in an electromagnetic field that pushes them along, similar to how gravity pulls at objects with mass. In the right conditions, electromagnetic fields can accelerate particles at near-light-speed.

On Earth, electric fields are often specifically harnessed on smaller scales to speed up particles in laboratories. Particle accelerators, like the Large Hadron Collider and Fermilab, use pulsed electromagnetic fields to accelerate charged particles up to 99.99999896% the speed of light. At these speeds, the particles can be smashed together to produce collisions with immense amounts of energy. This allows scientists to look for elementary particles and understand what the universe was like in the very first fractions of a second after the Big Bang. 

Download related video from NASA Goddard’s Scientific Visualization Studio

Magnetic fields are everywhere in space, encircling Earth and spanning the solar system. They even guide charged particles moving through space, which spiral around the fields.

When these magnetic fields run into each other, they can become tangled. When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection. The rapid change in a region’s magnetic field creates electric fields, which causes all the attendant charged particles to be flung away at high speeds. Scientists suspect magnetic reconnection is one way that particles — for example, the solar wind, which is the constant stream of charged particles from the Sun — is accelerated to relativistic speeds.

Those speedy particles also create a variety of side-effects near planets.  Magnetic reconnection occurs close to us at points where the Sun’s magnetic field pushes against Earth’s magnetosphere — its protective magnetic environment. When magnetic reconnection occurs on the side of Earth facing away from the Sun, the particles can be hurled into Earth’s upper atmosphere where they spark the auroras. Magnetic reconnection is also thought to be responsible around other planets like Jupiter and Saturn, though in slightly different ways.

NASA’s Magnetospheric Multiscale spacecraft were designed and built to focus on understanding all aspects of magnetic reconnection. Using four identical spacecraft, the mission flies around Earth to catch magnetic reconnection in action. The results of the analyzed data can help scientists understand particle acceleration at relativistic speeds around Earth and across the universe.

Particles can be accelerated by interactions with electromagnetic waves, called wave-particle interactions. When electromagnetic waves collide, their fields can become compressed. Charged particles bouncing back and forth between the waves can gain energy similar to a ball bouncing between two merging walls.

These types of interactions are constantly occurring in near-Earth space and are responsible for accelerating particles to speeds that can damage electronics on spacecraft and satellites in space. NASA missions, like the Van Allen Probes , help scientists understand wave-particle interactions.

Wave-particle interactions are also thought to be responsible for accelerating some cosmic rays that originate outside our solar system. After a supernova explosion, a hot, dense shell of compressed gas called a blast wave is ejected away from the stellar core. Filled with magnetic fields and charged particles, wave-particle interactions in these bubbles can launch high-energy cosmic rays at 99.6% the speed of light. Wave-particle interactions may also be partially responsible for accelerating the solar wind and cosmic rays from the Sun.

Download this and related videos in HD formats from NASA Goddard’s Scientific Visualization Studio

By Mara Johnson-Groh NASA’s Goddard Space Flight Center , Greenbelt, Md.

• The Inventory

How does radiation travel, and what kinds of damage can it do?

The nuclear disaster in Japan has left many people in danger of exposure to radiation and radioactivity. Some are simply fearful. What exactly is this radiation? And what threat does it pose? We've got some answers below.

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What is radiation?

Radiation has become a frightening word, but there's not necessarily anything dangerous about it. Visible light is radiation. Heat is radiation. Many news channels say that explosions or meltdowns 'leak' radiation into the atmosphere, but few say what exactly this 'radiation' is.

At least one of the explosions at the nuclear power plants has vented steam that contains radioactive materials. The nuclear power plant produces energy via fission of uranium-235, which means it splits uranium-235 into smaller atoms. Two types of these smaller atoms, cesium-137 and iodine-121, were found about sixty miles from the Fukushima Daiichi power plant. These radioactive materials can't be detected biologically, and so can be rubbed on the skin, eaten, or breathed in. These atoms break down into even smaller atoms, and their neutrons can change to protons. When this happens, they give off highly energetic gamma rays.

They also give out beta radiation. Beta particles are basically ejected electrons. Although they are not as energetic as gamma rays, they can cause problems if left on the skin for a prolonged time. Since radioactive materials can be ingested, beta particles can also be ejected inside the human body, where they can do a lot of damage.

What does radiation do?

Gamma rays are the highest energy rays in the electromagnetic spectrum. They are so energetic that they can travel great distances through the air or other material, and can pass through living material, which means any tissue in your body can be exposed to them. Gamma radiation is called ionizing radiation. When it encounters an atom, it can rip the electron right off, leaving an ion behind.

Once an atom becomes an ion, it reacts to the world around it in a different way. It can be attracted to certain things, repelled by others, or even join together with another atom. Since your living tissue is set up to work with non-ionic forms of atoms, introducing an ion into a part of the body where it doesn't belong has drastic consequences. Sometimes it only damages cells, but often it kills them off entirely. The effect depends on the level of ionization that the cells experience.

Low level ionization just causes some cells to be slightly damaged. This kind of damage is much the same as a sunburn. A sunburn is caused by relatively mild UV radiation, and only affects exposed areas of skin. Gamma radiation can burn any area of the body, and burns resulting from it are usually much more severe than sunburns. Although low level radiation exposure may not have any immediate effects, the damage can be done to the DNA inside your cells. This damage can cause your DNA to mutate, or behave abnormally. Ionization can also cause trouble when exposed cells divide - they may keep dividing without limit, causing cancer.

Higher levels of radiation have more immediate effects. Often the cells lining the intestines are damaged, and the body can't absorb water or nutrients. Bone marrow can also be affected. Without these, the body has a hard time fighting off external infections or diseases, and many victims of radiation die of infection. If, however, medical professionals are able to keep them alive and ward off infection, their bodies can recover.

Even higher levels of radiation exposure do damage to the vascular system. This damage is extensive, and can cut off the flow of blood to vital organs like the brain. When the damage is this severe, there is nothing that medicine can do to help the person affected. This level of radiation is almost always fatal.

Beta radiation is also very dangerous. The isotope iodine-131 is a particular worry because of its beta radiation and its placement within the body. Iodine-131 tends to bind to the thyroid. When it decays, the ejection of beta particles tends to harm fast-dividing cells. Since children are still growing, their cells are dividing particularly fast, and are very vulnerable. Too much exposure can cause thyroid cancer.

How does radiation travel?

Gamma rays travel like any other electromagnetic waves - cutting a fairly straight line through world. They can move through a vacuum, or through air or water. They can also cut through light elements like aluminum or most metals. Lead can cut down on gamma radiation, but it can't really stop it. One inch of lead will cut any amount of gamma radiation by half. Another inch will cut it by another half, and so on, and so on. Practically speaking, a few feet of lead will weed out pretty much any gamma radiation, but technically nothing can block all gamma rays from coming through.

Radioactive materials are also tough to contain. Although they can be measured, and scrubbed off, they are made of tiny, invisible atoms. Once they are released into the air, they can get blown by the wind or rain down on the land, get absorbed or eaten by plants and animals, adhere to matter, and scatter out through the world.

How much radiation is dangerous?

Radiation is difficult to measure. Exactly how much radiation anyone has been exposed to depends on the material they were exposed to, the distance they were from that material, and the time period they were exposed for. Absorbed doses of radiation are measured in a unit called a gray, which is a ratio of the amount of material absorbed to the mass of the matter that absorbed it. One gray is one joule of energy released into one kilogram of absorbing matter.

Many nuclear agencies prefer to measure the amount of radiation absorbed in sieverts. A sievert takes into account the type of energy released, as well as the way it was transmitted, and how vulnerable the absorbing tissue is to radiation. The typical exposure to radiation is measured in millisieverts, or thousandths of a sievert. Generally, people are exposed to about 3 millisieverts of radiation per year. An X-ray gives people 4 millisieverts, a CT scan gives 10. A dose of five hundred millisieverts ushers in radiation sickness, while exposure to one sievert gives the victim a ten percent chance of death within 30 days.

When the Fukushima plant was at its worst, it was giving out radiation at a rate of about 400 millisieverts per hour. That meant that most workers needed to be evacuated immediately. Fortunately, the level went down to about .6 millisieverts an hour. Levels of radiation throughout the area continue to be monitored.

Is there any way to avoid or treat radiation exposure?

The best way to avoid radiation exposure is to take steps to avoid materials contaminated with radioactive atoms. Distance is best - about 70,000 people have been evacuated from around the Fukushima plant. Clean food and clean water are also key. Families in the area of the plants are being encouraged to stay inside, to jam blankets and clothes in doors, and eat canned food.

The best defense people currently have only works against iodine-131. Since the thyroid absorbs iodine, and since it can be saturated with the stuff, children around the affected area are being given tablets containing normal iodine. When they eat them, the thyroid becomes filled with regular iodine, and can't take any more in, blocking the radioactive iodine from lodging in the body. Since iodine-131 has a half-life of eight days, the danger is temporary and can be staved off. Beyond that, there isn't much to be done about radiation exposure except treating the symptoms and getting screened for cancer regularly.

Although the danger is still great, most residents of Japan are being asked to stay calm. As are people elsewhere. The high level of radiation being given off by the Fukushima plant was a one time emission, and is unlikely to greatly affect residents of Tokyo, let alone residents of other countries. Radiation is to be avoided as much as possible, but it's also part of daily life.

Via Scientific American , NY Times , EPA , GM , CDC , Sievert Systems , and The Independent .

Top image by Pindyurin Vasily/Shutterstock

What is the light behaviour through different media?, Opaque, transparent and translucent objects

by Heba Soffar · Published April 2, 2015 · Updated April 16, 2024

Light is an electromagnetic wave , It is a form of energy that can travel freely across space, where the light rays travel through the vacuum present in the form of a wave when traveling in the transparent medium present in the form of particles.

The light waves

The light travels more slowly through the transparent materials than through a vacuum, so, the light bends as it enters or leaves the glass. Materials can be classified based on how light interacts with them, specifically how much light they allow to pass through. 

The light transmits through different media with variable degrees, So, Media can be classified according to their ability to allow the light to pass through, into transparent medium, translucent (semi-transparent) medium, and opaque medium.

The transparent medium

The transparent medium is the medium that permits most light to pass through them with little to no scattering or absorption, the objects can be seen clearly through a transparent medium, and images formed behind the object are undistorted. Examples of transparent materials include the air, the pure water, the glass cup, and the clear glass in a window are called transparent.

The transparent medium transmits all visible light, the color of a transparent object depends on the color of light that transmits, So if red light passes through a transparent object, the emerging light is red.

Although the water is a transparent medium, we can not see fish at the bottom of the sea as the thickness of the water at that point (the bottom) is large enough to prevent the light to pass through, B y increasing the thickness of the transparent medium, the quantity of light that passes through it decreases.

The translucent medium

The translucent medium is the medium that permits only a part of the light to pass through and absorbs the remaining part, the objects can be seen through the translucent medium less clearly than the transparent one. but the light is scattered or diffused so that objects behind the translucent material cannot be seen clearly. You can often tell that light is passing through the material because it may appear brighter on the other side, but the details of what is behind the material are obscured.

The materials like frosted glass, wax paper and some types of plastics, flint glass, and tissue paper are called translucent objects, when the light transmits the materials, only some of the light passes through them.

The light does not pass directly through the materials, It changes the direction many times and it is scattered as it passes through, so, we cannot see clearly through them.

The objects on the other side of a translucent object appear fuzzy and unclear, As the translucent objects are semi-transparent, some ultraviolet rays can go through them, this is why a person behind a translucent object can get a sunburn on a sunny day.

The opaque medium

The opaque medium is the medium that does not permit the light to pass through, and the objects can not be seen through the opaque medium.

Most materials are opaque, When the light strikes an opaque object none of it passes through, most of the light is either reflected by the object or absorbed and converted to heat.

Materials such as wood, stone, metals, plant leaves, foil paper, milk, books, cartoon, human skin, and black honey (molasses)  are opaque to visible light.

The cartoon is an opaque medium because it does not permit the light to pass through and the objects can not be seen behind it.

What is the light behaviour through different media?

Light behaves in fascinating ways when it travels through different media, which are materials like air, water, or glass.

Refraction of light in water : Light travels slowly in dense mediums like water or glass compared to a vacuum or air. This change in speed causes the light to bend its path, a phenomenon called refraction . This is why a straw appears bent when inserted into a glass of water .

Absorption: Some materials absorb certain wavelengths of light more than others. This is why a red shirt appears red – it absorbs most colors except red, which is reflected in our eyes, The absorbed light energy is converted to heat.

Reflection : When light strikes a surface, it can bounce back. This is reflection. The smoothness of the surface determines the type of reflection. Shiny surfaces like mirrors produce a mirror reflection, where the light ray bounces back at the same angle it hits. Rough surfaces cause diffuse reflection, scattering the light in various directions.

Scattering: Light can be scattered by particles in a medium. This is why the sky appears blue. Blue light has a shorter wavelength and gets scattered more easily by air molecules, reaching our eyes from all directions in the daytime sky. Red light, with a longer wavelength, scatters less and is what we see during sunrises and sunsets when the light travels through a thicker layer of the atmosphere.

Transmission: Light can pass through some materials entirely. This is called transmission. The amount of light transmitted depends on the transparency of the material. Transparent materials allow most light to pass through, while translucent materials allow some light through with some scattering.

The nature of light waves and analysis of white light

Dispersion of White light analysis, Normal prism and thin prism

Applications on the total reflection of light (Optical fibers, Reflecting prism, and Mirage)

Light wave properties, Analysis of white light, Spectrum colours and Light intensity

Types and Laws of light reflection, Regular and Irregular reflection of light

Light refraction effects, Law of light refraction, Mirage and Apparent positions of objects

Laws of light reflection, Plane mirrors, Spherical mirrors, Concave mirror and Convex mirror

Tags: 20 examples of translucent objects Difference between opaque transparent and translucent objects Different media Electromagnetic waves Light rays Light waves Opaque medium Opaque objects opaque objects examples opaque things Opaque transparent and translucent objects list opaque vs translucent vs transparent The light The materials The vacuum Translucent medium Translucent objects translucent objects examples translucent objects images transparent items Transparent medium Transparent objects transparent objects images Visible light What objects are opaque?

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Curious Kids: how does heat travel through space if space is a vacuum?

Senior Research Associate in Space and Planetary Physics, Lancaster University

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Curious Kids is a series by The Conversation , which gives children of all ages the chance to have their questions about the world answered by experts. All questions are welcome: you or an adult can send them – along with your name, age and town or city where you live – to [email protected]. We won’t be able to answer every question, but we’ll do our best.

How does heat travel through space if space is a vacuum? – Katerina, age ten, Norwich, UK.

What a great question!

First off, to understand what heat is, you need to know that everything you can touch or see is made up of tiny building blocks called atoms. Atoms are so small that you can’t even see them (except with some very special equipment ) – yet they make up all the matter in the universe.

If something is hot, it means that its atoms have lots of energy and are bouncing around. If something is cold, its atoms have much less energy and they stay quite still.

It’s true that space is a vacuum, which means that there isn’t much matter floating around out there. Space isn’t a perfect vacuum though. Even if we ignore the big stuff like stars, planets and comets, space is not completely empty.

In fact, the sun is constantly blowing matter, known as the solar wind , out into our solar system. This is part of what causes the beautiful light display we call the aurora.

Read more: Curious Kids: what causes the northern lights?

But the solar wind isn’t very dense - it has much, much fewer atoms in it than air, for example. This means it can’t carry much heat in it and so it can’t explain how the warmth from the sun reaches Earth.

There are three ways heat can be shared: conduction, convection and radiation. Let’s think about each of these in turn, to discover which one allows heat to travel through space.

Conduction is what scientists call the transfer of heat through touching. If you touch something warm, heat goes from it to you. If you touch something cold, heat goes from you to it.

Some materials, such as metals, are good conductors. Other materials, such as glass, are poor conductors, and are called insulators.

Heat can also be conducted in more than one step. For example, if you hold a metal spoon in a mug of hot tea, heat will be transferred from the tea to the spoon, and then from the spoon to your hand.

But we’re not touching the sun (and that’s a good thing too - its surface temperature is over 5,000°C!) and space is a vacuum so there isn’t anything to act as a spoon and conduct the heat. So we can rule out conduction.

Convection is the transfer of heat through the flow of fluids. Both liquids and gases can convect heat. Atoms will flow away from hot regions toward cooler regions, carrying their heat and energy with them.

If you’ve ever been in a bath that has started to go cold, and then turn the hot tap on, you’ll feel the hot water convect from the tap further into the bath.

The hot atoms will then bump into colder atoms, sharing their heat through conduction, until the bath becomes an even temperature.

But because space is a vacuum, there are no liquids or gases to convect heat away from the sun, all the way to Earth. So we can rule out convection.

Hot bodies of matter such as the sun – and even our own human bodies – give off heat. As the matter’s atoms move and vibrate they give off, or “radiate”, electromagnetic energy – this is called “thermal radiation”.

Electromagnetic energy comes in a range, or spectrum, of types - some of these we can see: they make up the rainbow of “visible light”. Other types that we cannot see exist too, such as the infrared energy our hot bodies radiate and microwave energy we use to cook food.

Unlike conduction and convection, radiation does not need matter to transfer heat. Energy is radiated from the sun, through the vacuum of space at the speed of light. When this energy arrives at Earth, some of it is transferred to the gases in our atmosphere.

Some of it passes through and heats up the atoms on the earth’s surface. Some will even be absorbed by your skin .

The ground soaks up the energy from the sun’s radiation, and this causes it to give off heat, too. Some of this heat is conducted – like when the hot sand on the beach burns your feet in the summer. Some is convected through wind and ocean currents , and some of it is radiated back into the atmosphere , or even outer space.

More Curious Kids articles, written by academic experts:

What makes a shooting star fall? - Katelyn, age seven, Adelaide, Australia.

What causes the northern lights? – Ffion, age 6.75, Pembrokeshire, UK.

How is water made? – Clara, age eight, Canberra, Australia

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