Light

Light gives us fuels. The energy in the sunlight that shone on the earth millions of years ago was stored by plants. After these plants died, they were changed into coal, natural gas, and oil. Today, we use the energy in these fuels to produce electricity and to operate machines.

Light from the sun also heats the earth. Without the sun's light, the earth would soon become so cold that nothing could live on it. Even if we burned all our fuels, we could not keep the earth warm enough for life to exist. For more information on light and energy from the sun, see the articles on Solar Energy and Sun.

People have found ways of making and controlling light in order to see when there is no sunlight. At first, they produced light with campfires and torches. Later, they developed candles, oil lamps, gaslights, and electric lights.

People make and use light for many other purposes than to see by. For example, the pictures on a television screen consist of spots of light. Using scientific instruments, people can study light itself and learn much about the universe. For example, the light from distant stars can tell scientists what the stars are made of. It can also tell them if the stars are moving toward or away from the earth and how fast they are moving. See Red Shift.

What is light? This question has been a puzzle for centuries. People once thought light was something that traveled from a person's eyes to an object and then back again. If anything blocked the rays from the eyes, the object could not be seen. Since the 1600's, scientists have made many discoveries about light. They have learned that light is a form of energy that can travel freely through space. The energy of light is called radiant energy. There are many kinds of radiant energy, including infrared rays, radio waves, ultraviolet rays, and X rays. We can see only a tiny part of all the different kinds of radiant energy. This part is called visible light or simply light.

This article describes where light comes from, the nature of light, and how light behaves when it comes in contact with various materials. The article also tells how light is measured and discusses the important scientific discoveries about light. By building the Science Project included in this article, you can experiment with the behavior of light. For more information on how people use light for seeing, see Lighting.

How light is produced. All light comes from atoms. It is produced by atoms that have gained energy either by absorbing light from another source or by being struck by other particles. An atom with such extra energy is said to be excited. Ordinarily, an atom stays excited only briefly. It de-excites by giving up its extra energy. It can either run into another atom to lose the energy, or it can emit (give off) light. The light then carries away the extra energy. The amount of energy needed to excite atoms and the amount of energy the atoms emit as light varies for different kinds of atoms.

Light is usually described as a wave, shaped much like a water wave that moves across a lake. But light can also be described as a small particle, called a photon. Each photon moves in a straight line, much as a pool ball does. In both descriptions, the light has energy. The amount of energy that is carried by the wave or photon largely determines the color of the light. For example, suppose you see a red apple on a blue chair. Each photon from the apple has less energy than a photon from the chair.

One way to excite atoms so that they emit light is by heating them. A poker may be heated until it is white-hot. Because of the heating, the atoms at the poker's surface collide violently with each other. When they collide, they excite one another. Each atom quickly emits its extra energy as light but is almost immediately re-excited by another collision. These collisions produce such a variety of states among the atoms that the photons released have a wide range of energies. The combination of all the resulting colors is white light. As the poker cools, fewer atoms are excited to high energies, and so the atoms emit fewer photons with the higher energies of blue light. Since red light is still being emitted, the cooling poker looks red.

Other sources of light. Many substances gain energy and emit light without being heated very much. They do this through a process called luminescence. Some luminescent materials glow in the dark long after they have received extra energy. They are said to be phosphorescent. Their atoms stay excited for some time before they de-excite and emit light. Certain phosphorescent materials are used in the markings that glow on watch faces (see Phosphorescence). Other luminescent materials emit light only during their exposure to exciting energy. They are said to be fluorescent (see Fluorescence; Fluorescent Lamp).

Fireflies and a few other types of organisms emit light by a process called bioluminescence. In this process, chemicals within the organisms combine to produce a different chemical that has excited atoms. When the atoms de-excite, they emit photons.

The sun shines because nuclear reactions between hydrogen atoms within its core produce a tremendous amount of energy. Photons and other kinds of particles carry the energy to the sun's surface. At the surface, these particles excite atoms that then de-excite by emitting light. The earth receives part of that light. All stars emit light by this process.

An aurora such as the northern lights is an emission of light by molecules of air. When high-speed particles arrive at the earth from large eruptions on the sun, they crash into the air molecules. These collisions excite the molecules with extra energy. The molecules then release the energy by giving off light. When the collisions occur at night, the light emitted may be bright enough to be seen.

A laser is a device that produces a powerful, narrow beam of light in which all the photons have the same energy and travel in the same direction. Lasers serve as tools in scientific research, surgery, and telephone communications. They also have many industrial and military uses.

If light is a wave, then what waves? Water waves are easier to explain. They travel across the surface of the water while the water itself only moves up and down. To scientists of the 1800's, light seemed stranger than water waves because it travels through space from the sun and other stars to the earth. They assumed that light waves must also travel through some kind of material, just as water waves travel through water. Although scientists had no evidence of this material, they called it the ether. By the late 1800's, scientists had concluded that light waves consist of regions of force known as electric fields and magnetic fields.

A simple model of a light wave begins with a ray (a straight line) that shows the direction of the light's travel. Along the ray and perpendicular (at right angles) to it, short arrows represent the electric field. Some arrows point upward from the ray and other arrows point downward from it. They vary in length so that the overall pattern of the tips of the arrows looks like a wave. Arrows representing the magnetic field also resemble a wave, but these arrows make right angles to the arrows that represent the electric field. These patterns move along the ray. They are the light.

By the early 1900's, experiments had shown that scientists finally had to give up the idea of an ether. Many scientists realized that a wave of light, as a regularly varying pattern of electric and magnetic fields, can travel through empty space.

Light waves resemble other types of waves in some features, including wavelength, frequency, and amplitude. The wavelength is the distance along a straight line from one crest (peak) of the wave to the next. The frequency of a wave is the number of times each second that crests pass a stationary checkpoint. The amplitude of a wave is the greatest distance of a crest or trough (low point) from the ray.

A simple relation exists between a wave's frequency and wavelength: the higher the frequency, the shorter the wavelength. A wave's energy corresponds to its amplitude. The greater the amplitude, the more energy the wave has. The energy of a light wave also corresponds to its frequency. The wavelength determines the color of the light.

Photons. In 1905, the German-born physicist Albert Einstein proposed a model of light just as useful as the wave model. In some experiments, light behaves as though it is a particle. We now call this type of particle a photon. In Einstein's model, a ray of light is the path taken by a photon. For example, when a flashlight sends a beam of light across a dark room, the beam of light consists of a great many photons, each traveling in a straight line.

Is light a wave or a particle? Seemingly, it cannot be both because the two models are so different. The best answer is that light is strictly neither. In some experiments light behaves like a wave, and in others it behaves like a particle.

Unlike other kinds of waves, light waves in a vacuum have one speed, and that speed is the fastest that anything can travel. Scientists do not understand why this is true. The fact that light in a vacuum has only one speed forms one of the foundations of Einstein's theory of relativity.

When light enters a material, it continually runs into atoms that delay its travel. But between atoms, light travels at its normal speed.

Electromagnetic waves. Because light consists of electric and magnetic fields, it is called an electromagnetic wave. The term light commonly refers to just those electromagnetic waves that we can see. For light to be visible, it must have a wavelength within a certain narrow range of values called the visible spectrum. Violet light has the shortest wavelength that is visible. Red light has the longest. Between them lie all the other colors of the spectrum, each with its own wavelength. Seen together at the same time, the colors appear as white light. Sunlight is white because it has all the colors. However, when it passes through a specially shaped transparent solid called a prism, the different colors separate and can be seen.

The visible spectrum forms only a small part of the full range of electromagnetic waves. Waves that have wavelengths slightly too short to be seen are called ultraviolet rays. They cause suntan, sunburn, and skin cancer. Waves with somewhat shorter wavelengths than ultraviolet rays are called X rays. These rays can penetrate a person's body. Doctors and dentists use them to "see" inside the body. Gamma rays have even shorter wavelengths than X rays. They result from nuclear reactions, such as those in the sun.

Waves with wavelengths slightly longer than those of red light are called infrared rays. When you stand in bright sunlight or in front of a fire, you feel warm largely because of the infrared light shining on you. Microwaves and radio waves have longer wavelengths than infrared waves. A microwave oven shines microwaves on food to warm it. A police officer's radar unit shines microwaves onto an approaching car to measure its speed. Radio and television stations broadcast programs by sending radio waves.

Sunlight spread into its different colors by a prism creates a continuous spectrum. From violet to red, the spectrum blends smoothly from one color to the next. Many other sources of light do not produce a continuous spectrum. For example, a street lamp may produce bright yellow, blue, and a few dimmer colors, but it also has dark regions in its spectrum. The colors are produced by certain atoms in the gas inside the lamp. For example, the yellow comes from sodium atoms. Each type of atom can produce only certain colors.

Scientists can learn what kinds of atoms make up a light source by observing what colors are present in the light. They direct the light through an instrument called a spectrometer to separate the colors. The spectrometer may be a simple prism or it may be a more complicated device.

Sometimes a spectrum contains gaps because the light from a source has traveled through a gas that absorbed certain colors. For example, when sunlight is sent through a high-quality spectrometer, its spectrum has thousands of such gaps. The light produced within the sun must travel through the outer atmosphere of the sun to reach the earth. Each type of atom in the sun's atmosphere absorbs certain colors. By noting which colors are removed, scientists are able to determine what kinds of atoms are in the atmosphere of the sun. See Spectrometer.

Reflection, refraction, and absorption. When a ray of light reaches a surface between two types of materials, such as air and glass, several things can happen. Some of the light may reflect from the surface, while some may pass through the surface. The light that enters the second material may refract (change its direction). In addition, some light may be absorbed by molecules on the surface or within the second material.

A transparent material lets light rays pass through it without mixing them up. You can see through such material. A translucent material also allows rays to pass through it, but it mixes them up so that you cannot see clearly through the material. An opaque material blocks all light.

The reflection of a ray from a surface resembles the bounce a pool ball takes at the edge of a pool table. Imagine a line perpendicular to the reflecting surface. Such a line is usually called the normal. The angle between the path of an incoming ray and the normal is called the angle of incidence. The reflected ray makes the same angle to the normal as the incoming ray, but on the other side of the normal. Reflection works this way even when it involves rough surfaces. Wherever a ray reflects from a surface, it has an equal angle to the normal at that spot as it had before the reflection.

When light reflects from a smooth surface, all of its rays reflect in the same direction. When light reflects from a rough surface, the rays reflect in many directions because the normals at all spots on the surface point many ways. Thus, you can see your image in a mirror, but not in a sheet of paper. See Reflection.

When light passes through a surface, its speed changes. This happens because the light must travel through a different kind of molecule than it passed through before. For example, if light passes from air into glass, it slows because the glass molecules are more densely packed than the air molecules. If the light enters at any angle except a right angle, the change in the light's speed changes its direction of travel. In other words, the light refracts.

When a ray passes from air into glass, it bends towards the normal at the surface. The amount of bending depends partly on the type of material the ray enters. Different types of glass and plastic refract light by different amounts. Diamond refracts light much more than either glass or plastic does.

To observe refraction, place a pencil in a glass of water and then look at the pencil from the top and one side. The pencil appears bent at the water surface. The light from the top part of the pencil comes directly to your eyes. The rays from the bottom part pass through the surface between the water and the air. There the rays refract, and so they seem to have come from a pencil bottom bent from the pencil top. See Refraction.

Opaque materials absorb certain colors of light. For instance, a red book cover exposed to white light looks red because molecules on its surface absorb all the other colors in the light. Transparent materials also absorb certain colors if they contain dyes or pigments.

Scattering describes what happens when light rays strike atoms, molecules, or other individual, tiny particles. These particles send the rays of light off in new directions--that is, they cause the rays to scatter. Most of a clear sky appears blue because air molecules scatter more blue rays toward us than they do the other colors in sunlight. When the sun is near the horizon, it looks orange or red because the light reaching us has lost so much of its other colors by scattering.

On a clear day, the ocean appears blue because of two processes: (1) The ocean's surface reflects some of the blue light from the sky toward the observer. (2) Light coming directly from the sun enters the water. The water molecules then scatter more blue rays toward the observer than they do the other colors in sunlight.

Interference. In many cases, light can be thought of as being a wave with crests and troughs. When two light waves cross through the same spot, they interfere with each other--that is, they add to or subtract from each other. Suppose that whenever a crest of one wave passes through the spot, so does a crest of the other wave. The two crests add together to give a larger crest. This process, called constructive interference, gives brighter light than either wave would have separately. Suppose instead that whenever a crest of one wave crosses through the spot, a trough of the other wave also does. The trough reduces the height of the crest, leaving the spot dim or even dark. This process is called destructive interference.

The fact that light can interfere to produce brightness or darkness provides the strongest argument for the wave model of light. In the early 1800's, the English scientist Thomas Young showed the wave nature of light by sending a light beam through two narrow slits. The light that emerged from the slits then reached a screen. If light were not a wave, only two narrow, bright strips of light--one from each slit--would have appeared on the screen. But, in fact, the light emerging from each slit spread and overlapped the light emerging from the other slit.

This light filled the screen with bright and dark lines called fringes. Bright fringes occurred where the two waves arrived crest-on-crest to give constructive interference. Dark fringes occurred where the two waves arrived crest-on-trough to give destructive interference. See Interference.

Diffraction. In Young's experiment, the light passing through each slit spread. This type of spreading is called diffraction. Like interference, it results from the fact that light behaves as a wave. A light wave spreads slightly when it travels through a small opening, around a small object, or past an edge. Water waves also spread, but the openings and objects that cause them to spread must be much larger than those for light.

Diffraction of light can be a nuisance. Suppose you attempt to see a very small object by using a high-quality microscope. As you increase the magnifying power to see the object more and more closely, the object's edges begin to blur. Each edge blurs because the light passing by the edge on its way to the eye diffracts.

However, diffraction serves a purpose when a device called a diffraction grating is used to study the colors in a light beam. The grating consists of thousands of thin slits that diffract light. Each color in the light diffracts by a slightly different amount. The spread of colors can be large enough to make each color visible. A grating used with a telescope can separate the colors in the light from a star, enabling scientists to learn what materials make up the star. See Diffraction.

Dispersion is the spreading of light into its colors. The dispersion of white light separates the colors of the full visible spectrum. One way to disperse a light beam is to send it through a prism. The different colors refract to different extents. Thus, the colors spread. Diffraction and scattering can also disperse light.

Polarization involves the oscillations (regular variations in strength) of the electric fields that make up a light wave. The directions of the oscillations may be represented by arrows. In most of the light we see, the arrows point in many directions perpendicular to the ray's path. Such light is unpolarized. But few of the arrows remain when light passes through certain types of sunglasses, reflects from surfaces at certain angles, or scatters from air molecules. If these arrows all point in one direction or just opposite it, the light is polarized. Suppose that when sunlight reflects from a road to you, its arrows point only to your left or right. You can block it by wearing sunglasses with polarizing filters. They block light oscillating left or right. See Polarized Light.

Chemical effects of light. The energy of light can chemically change the surfaces of materials absorbing it. For example, light chemically changes the molecules of silver grains on photographic film so that an image can be recorded on it. Strong light can fade colored fabrics by chemically changing their dyes. Light changes the chemistry of the eye's retina, so that the retina produces signals about sight (see Eye). Green plants need light for photosynthesis, the chemical process by which they make food (see Photosynthesis).

Photoelectric and photoconductive effects. When certain materials absorb light, the light's energy frees electrons from atoms on the materials' surface. In some devices, these freed electrons can flow through a circuit as electric current. Solar cells and other electric eyes operate by means of such photoelectric effects (see Electric Eye). Some materials called photoconductors become better conductors of electricity when light shines on them.

The frequency of any wave equals the ratio of the wave's speed to its wavelength. Frequencies are measured in units called hertz. A wave has a frequency of one hertz if one crest passes a checkpoint each second, and the wave has a frequency of 100 hertz if 100 crests pass a checkpoint each second. Light travels in a vacuum at nearly 300 million meters per second. Because visible light has a short wavelength and a high speed, it has a high frequency. For example, violet light has a frequency of 750 trillion hertz.

The brightness of light. Scientists use various units to measure the brightness of a light source and the amount of energy in a beam of light coming from that source.

The amount of light produced by any light source is called the luminous intensity of that source. The standard unit used to measure luminous intensity is the candela. For many years, the luminous intensity produced by a certain size candle made from the oil of sperm whales served as the standard. The unit was called a candle. However, the sperm whale candle did not provide an easily used standard for the measurement of light. One candela is now defined as the amount of light given off by a source emitting at a specific frequency (540,000,000,000,000 hertz) and at a specific intensity (1/683 watt per unit of area called a steradian).

The intensity of a light source in candelas does not indicate how bright the light will be when it reaches the surface of an object, such as a book or a desk. Before we can measure illumination (the light falling on a surface), we must measure the light traveling through the space between the source and the object. We can measure a beam of light with a unit called the lumen. To see how the lumen is measured, imagine a light source placed at the center of a hollow sphere. On the inside surface of the sphere, an area is marked off equal to the square of the radius of the sphere. For example, if the radius is 1 foot, the area marked off is 1 square foot. If the light source has a luminous intensity of 1 candela, the marked area will receive a luminous flux (rate of light falling on it) of 1 lumen.

In the customary system of measurement, engineers measure illumination in units called foot-candles. An illumination of 1 foot-candle is produced by 1 lumen of light shining on an area of 1 square foot. The metric system uses a unit called the lux. An illumination of 1 lux is produced by 1 lumen of light shining on an area of 1 square meter. See Foot-Candle.

The intensity of light falling on a surface varies inversely (oppositely) with the square of the distance between the source and the surface. That is, if the distance increases, the illumination decreases by the square of the distance. This relationship is called the inverse square law. If a surface that receives 1 lux of light at a distance of 1 meter from a source is moved 2 meters from the source, that surface will then receive 1/2-squared, or 1/4, lux of light. This happens because light spreads out from its source.

The speed of light. Although light seems to travel across a room the instant a window shade is raised, it actually takes some time to travel any distance. The speed of light in empty space--where atoms do not delay its travel--is 186,282 miles (299,792 kilometers) per second. This speed is said to be invariant because it does not depend on the motion of the light's source. For example, light that is emitted by a rapidly moving flashlight has the same speed as light that is emitted by a stationary flashlight. Scientists do not know why this is true, but the fact is one of the foundations of Einstein's theory of relativity.

From ancient times, people argued about whether the speed of light is limited or infinite. During the early 1600's, the Italian physicist Galileo devised an experiment to measure the speed of light, and so settle the argument. Galileo sent an assistant to a distant hill with instructions that the assistant should open the shutter of a lantern when he saw Galileo on another hill open the shutter of his lantern. Galileo reasoned that because he knew the distance between the hills, he could find the velocity of light by measuring the time between opening his shutter and seeing the light of the second lantern. Galileo's thinking was sound, but the experiment failed. The velocity of light is so great that he could not measure the short time involved.

About 1675, the Danish astronomer Olaus Roemer came upon evidence which proved that light travels at a finite (limited) speed. While working in Paris, Roemer observed that the intervals between the disappearance of some of Jupiter's moons behind the planet varied with the changing distance between Jupiter and Earth. Roemer realized that the finite velocity of light caused these differing intervals. Roemer's observations indicated that light traveled at a speed of 226,000 kilometers per second. This figure was within 25 per cent of the actual velocity.

In 1926, the American physicist Albert A. Michelson made one of the first precise measurements of the velocity of light. He used a rapidly rotating mirror that reflected a beam of light to a distant reflector. The returning beam was then reflected back to the observer by the rotating mirror. Michelson adjusted the speed of the mirror until the mirror turned to the correct angle during the time the light traveled to the reflector and back. The speed of the mirror indicated the velocity of the light. Michelson actually used several mirrors on a drum so that the angle the drum had to turn while the light traveled out and back was small. He measured the speed of light at 299,796 kilometers per second. This measurement had a probable error of less than 4 kilometers per second.

About the same time that Newton proposed his theory of light, the Dutch physicist and astronomer Christiaan Huygens suggested that light consists of waves. He proposed the wave theory to explain the behavior of light. The corpuscular and wave theories appear to be completely opposite, and scientists argued about them for about 100 years. Then, in the early 1800's, the English physicist Thomas Young demonstrated the interference of light. He showed that two light beams cancel each other under certain conditions. Water waves also behave this way. Because it is hard to understand how interference could occur with particles, most scientists accepted Young's experiment as proof of the wave theory of light.

The electromagnetic theory. In 1864, the British physicist James Clerk Maxwell proposed the mathematical theory of electromagnetism. According to this theory, the influence that changing electric fields and magnetic fields have on one another allows for the travel of waves. Maxwell's theoretical waves had the exact mathematical properties that had been measured for light. The vibrating electric charges that produce light are the electric charges in the atom. Atomic physicists had already shown that these vibrating electric charges exist. Maxwell's work gave the wave theory of light a solid foundation.

Maxwell's electromagnetic theory also did away with an idea that had stood in the way of scientists' acceptance of the wave theory for more than a century. Scientists felt they had to find the medium (material) through which light waves travel. They reasoned that if light travels as waves, there must be something for them to travel through, just as sound waves need air to travel through. But for light, this something could not be matter, because light can travel in a vacuum. To get around this difficulty, scientists suggested that the medium light traveled through was the ether.

All attempts to observe or measure the properties of the ether failed. Scientists became increasingly convinced that the ether did not exist. Experiments conducted by Albert Michelson and the American physicist Edward Morley in 1887 helped destroy the ether theory.

Quantum mechanics. In 1900, the German physicist Max Planck discovered an equation that matched experimental data about the emission of light by a hot surface. Planck could not explain why the equation worked. But he realized that it predicted that the tiny emitters of light on the surface can have only certain values of energy. When energy is restricted to certain values, it is said to be quantized.

In 1905, Einstein revealed that light itself is quantized. Einstein reasoned that if light emitters can have only certain values of energy, then the energy they emit as light will retain its quantized character. The light comes in tiny packets of energy that are known as quanta. The concept of light as quantized energy explained how light behaves as a particle in certain experiments, instead of as a wave. These particles of light came to be called photons.

In 1913, the Danish physicist Niels Bohr proposed that the energy of atoms was also quantized. When energy is given to an atom, either by a collision or by shining light on it, the atom can accept only certain values of energy. In this way, the atom becomes excited. When it de-excites, it must get rid of the extra energy. One way it can do this is by emitting a photon that carries the energy away. Each type of atom accepts a different set of energies. Thus, when atoms emit light, the photons from one type of atom differ in energy from the photons from other types of atoms.

A field of physics known as quantum mechanics is the study of how atoms and light are quantized. It involves the fact that light and matter behave as waves in some experiments and as particles in other experiments.

Contributor: Jearl Walker, Ph.D., Prof. of Physics, Cleveland State Univ.

Related articles include:

Biographies
Einstein, Albert; Foucault, Jean Bernard Leon; Huygens, Christiaan; Maxwell, James Clerk; Michelson, Albert Abraham; Newton, Sir Isaac; Planck, Max Karl Ernst Ludwig; Raman, Sir Chandrasekhara Venkata.

Other related articles
Aberration; Angstrom; Aurora; Bioluminescence; Candela; Color; Diffraction; Electric Eye; Electric Light; Electromagnetic Waves; Ether; Fiber Optics; Fluorescence; Foot-Candle; Infrared Rays; Interference; Laser; Lens; Light Meter; Lighting; Luminescence; Microscope; Mirage; Newton's Rings; Optics; Phosphorescence; Photochemistry; Photosynthesis; Polarized Light; Quantum Mechanics; Rainbow; Reflection; Refraction; Shadow; Spectrometer; Sun; Telescope; Ultraviolet Rays; Waves; Zeeman Effect.

What are photons?

Why is light strictly neither a wave nor a particle?

What property of a light source is measured in lumens?

What is diffraction? Interference? Polarization?

How does the heating of atoms cause them to give off light?

Why did scientists suggest that light traveled through a medium called the ether?

What is the visible spectrum?

Why does a pencil in a glass of water appear bent at the water surface?

What theory did the British physicist James Clerk Maxwell propose that strengthened the wave theory of light?

Burnie, David. Light. Dorling Kindersley, 1992.

Taylor, Barbara. Bouncing and Bending Light. Watts, 1990.

Feynman, Richard P. QED: The Strange Theory of Light and Matter. Princeton, 1985.

Gardner, Robert. Experimenting with Light. Watts, 1991.

Sobel, Michael I. Light. 1987. Reprint. Univ. of Chicago Pr., 1989.

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