Introduction to Light
Light, the form of radiant energy that makes vision possible. Since sight is the most important of the senses, light furnishes most of the information we have about our surroundings. Even more important, light makes life possible. In the process called photosynthesis, plants and some other organisms use the energy of sunlight to make food from water and carbon dioxide. Animals obtain their food by eating such organisms or by eating animals that eat such organisms. The energy of sunlight contained in an animal's food is released when the food is oxidized (combined with oxygen) in the animal's body. Thus most living things receive their energy, directly or indirectly, from light.
The energy of sunlight is also stored chemically in wood, coal, petroleum and other fuels that are the remains of plants and animals. When these fuels are burned (rapidly combined with oxygen), part of this energy is given off as heat, and part as light.
Light influences living things in many other ways. Plants and some primitive animals move either toward or away from light. Most flowering plants open their flowers in response to light. Flowers typically bloom only when the hours of daylight are a certain length. The response of living things to the hours of daylight is called photoperiodism. Among other things, photoperiodism determines when plants prepare to shed their leaves, and when certain animals breed, hibernate, or migrate.
Light produces electricity in some substances, such as those used in solar batteries. In such materials as photographic films and papers, light produces chemical changes.
The Nature of Light
The nature of light has puzzled humans since the most ancient times. One belief was that it consisted of something that shot out of the eye and made things visible when it struck them. Aristotle refuted this idea when he asked why, if the eye is the source of light, it is impossible to see in the dark.
About 1670 Sir Isaac Newton suggested that light consists of tiny particles that shoot out from luminous bodies, such as the sun and fires. This corpuscular theory could explain some facts about light but not others. In 1678 Christian Huygens suggested the wave theory. Work in the early 19th century by Augustin Jean Fresnel, Thomas Young, and others showed that almost all facts about light could be explained in terms of waves, and the wave theory of light became accepted.
Scientists could not conceive of waves as moving in nothing, so they decided that light must travel through a substance they called the ether. The ether was an invisible, intangible substance that pervaded all space. Light was regarded as being waves in ether.
In the 1870's James Clerk Maxwell showed that the ether was not necessary to explain the wave theory of light. Light and other forms of radiation could be explained as alternating pulses of electricity and magnetism that pushed each other out into space as electromagnetic waves.
Max Planck, a German physicist, in 1900 showed that some facts about radiation cannot be satisfactorily explained in terms of waves but only in terms of packets of energy called quanta, or photons. Thus the quantum theory came into existence.
Some properties of light can be explained only in terms of photons, though some others can be explained only in terms of waves.
Sources of Light
Natural Sources of Lightinclude the sun and other stars. In stars, hydrogen atoms are fused to form helium atoms. Vast quantities of energy are released in the fusion process, and part of this energy takes the form of light. The Aurora Borealis and Aurora Australis (Northern and Southern Lights) occur when streams of electrons and other electrically charged particles shot out from the sun excite gases in the upper atmosphere.
Meteors give off light when heated by friction as they pass through the atmosphere. The light given off as a result of heat is called incandescent light. Fireflies and many other living organisms give off light. The production of light by living organisms is called bioluminescence. Substances that emit light when struck by light, ultraviolet rays, or other forms of radiation are said to be fluorescent. Those that continue to glow after the radiation has stopped are called phosphorescent.
Artificial Sources of Lightinclude matches, candles, incandescent electric lamps, and other forms of incandescent light. In fluorescent lamps, phosphorescent substances glow when struck by ultraviolet rays. In neon lights and similar lights, gases glow when bombarded with electrons. Electroluminescence is produced by phosphorescent substances that glow when excited by an electric current.
Properties of Light
Light waves differ from radio waves, X rays, and other electromagnetic waves only in their length. Light waves are of the right length to affect the optic nerves to make sight possible. Other electromagnetic waves are either too short or too long to do this.
SpeedLight travels at 299,792 kilometers per second (186,282 miles per second). First to measure the speed of light was Olaus Roemer, in 1675, using eclipses of Jupiter's moons.
A French physicist, Armand Fizeau, in 1849 obtained a more accurate measurement by measuring the time it took for a beam of light to travel to a mirror 8,633 meters (about 5 13 miles) away and then return to a mirror at the light source. Albert Michelson, a United States physicist, used a refinement of Fizeau's method and got a still more accurate measurement in 1923. Later, more precise measurements were made by determining the speed of radio waves, and by complicated methods involving lasers and cesium-beam atomic clocks.
It is important for scientists to know the exact speed of light because it is involved in many significant calculations in physics, particularly in Einstein's equation for the interchangeability of energy and matter. The speed of light is believed to be the maximum speed attainable by any material object.
RefractionLight travels fastest in a vacuum. It moves somewhat more slowly through air, still slower through water, and slower yet through glass. When light passes from one substance into another its change of speed causes it to be bent, or refracted. Thus when a spoon is placed in a glass of water, the handle appears bent at the surface of the water.
Use is made of refraction by lenses and prisms to make telescopes, microscopes, cameras, eyeglasses, and other optical devices. The refraction of light by air causes mirages. The twinkling of stars and the halo often seen about the moon are other results of refraction.
ReflectionPart of the light that falls on a surface bounces, or is reflected. Regular reflection occurs in mirrors and smooth, polished, metal surfaces. Diffuse reflection occurs on rough surfaces. Use is made of regular reflection in mirrors, reflecting telescopes, and the reflectors used in spotlights, automobile headlights, and other lighting applications.
Interferenceoccurs when two waves meet. The waves then reinforce each other when their crests meet, and cancel each other when the trough of one meets the crest of the other. Interference in light waves occurs when light is passed through two tiny holes onto a screen. The effect of interference is to produce alternating bands of light and shade. Interference occurs only in waves. This fact was one of the strongest arguments in favor of the wave theory of light.
Diffractionis the bending of waves as they pass about obstacles and their spreading after passing through openings. Light is diffracted when it passes through a tiny hole or through a diffraction grating, a piece of glass in which closely spaced parallel lines are cut. When white light is diffracted, it is separated into a spectrum consisting of the colors that form white light. Diffraction of light is used in chemical analysis by means of spectroscopes.
PolarizationThe ripples of a light wave move at right angles to the path of the wave. A beam of light is made up of a vast number of waves, and these normally vibrate at many angles to one another. Some move up and down, others move sideways, and still others move at various angles between the vertical and the horizontal. A crystal of tourmaline, however, absorbs all waves except those that vibrate in one plane. Light that vibrates in one plane is called polarized light. The process of producing polarized light is called polarization. Polarized light has important uses in chemical analysis. Polarized glass, used in sunglasses, filters for cameras, and other optical devices, reduces glare.
Intensity of Light
Light ordinarily travels in straight lines. One proof of this is the sharp shadows of objects lighted by small sources of light. As it travels, light spreads and grows dimmer. The area covered by a beam of light increases with the square of the distance from the source. The intensity of illumination decreases at the same rate. Thus a book held four feet from a light source receives only 116 of the light it would receive at one foot. The measurement of light intensity is called photometry. Photometry is employed in determining the proper lighting of rooms, in photography, and for a number of other purposes.
Books about Light
Bachor, Hans A. A Guide to Experiments in Quantum Optics (Wiley, 1998).
Silverman, Mark P. Waves and Grains: Reflections on Light and Learning (Princeton University Press, 1998).
Waldman, G. Introduction to Light (Dover Publications, 2002).
Nu Energy Horizons Staff. Radiant Energy Power Sources (Nu Energy Horizons, 2002).
For Younger Readers
Levine, Shar. The Optics Book: Fun Experiments with Light, Vision and Color (Sterling, 1999).
New Dimension Media Staff. Light Waves (New Dimension Media, 1997).
Phillips, David. Light up Your Life (Portland Press, 1997).
Shapiro, Irwin I., Marvin C. Grossman, and R. Bruce Ward. ARIES Exploring Light and Color: Filters, Lenses, and Cameras (Charlesbridge, 2000).
The intensity of light can be amplified (made greater) if the energy of a light signal is increased without changing the frequency or wave length. Ordinary optical devices, such as burning glasses and telescopes, do not amplify but merely concentrate light rays or increase the apparent size of the light source. True amplification can be achieved by lasers and other electronic devices in which light rays stimulate the atoms in a crystal or other medium. In light amplified by a laser, light rays are parallel to one another, instead of fanning out in all directions as from an ordinary source.
Light and Vision
Objects are visible because they give off or receive light. Objects that give off light are called luminescent, and those that receive light are said to be illuminated. Some illuminated objects transmit light, and nearly all absorb and reflect light. Transparent objects transmit most of the light that reaches them, thus making it possible to see objects through them. Clear glass is an example. Translucent substances, such as frosted glass and waxed paper, are partially transparent; they diffuse light, and thus objects cannot be seen through them. Opaque substances transmit no light.
The colors of objects depend on how they reflect and absorb light. White light is made up of violet, blue, green, yellow, orange, and red light. When passed through a prism, it separates into these colors. A substance that reflects all the colors of white light is white. A substance that absorbs all these colors is black. Most objects absorb some colors and reflect others. They are called by the colors they reflect. Thus an object that absorbs all colors except blue is called blue.
Vision depends on how light affects the eye, and this depends on the condition of the eye itself.