One
of the most familiar and important forms of energy is light. Nothing is
visible to humans when light is totally absent. But light is even more
important for other reasons. Many scientists believe that millions of
years ago light from the sun triggered the chemical reactions that led
to the development of life on Earth. Without light the living things
now on Earth would be unable to survive. Light from the sun provides
energy for life on Earth. Plants change the energy of sunlight into
food energy. When light rays strike a green plant, some of their energy
is changed to chemical energy, which the plant uses to make food out of
air and minerals. This process is called photosynthesis. Very nearly
all living organisms on Earth depend directly or indirectly on
photosynthesis for their food energy.
Some
of the energy of sunlight is absorbed by the Earth's atmosphere or by
the Earth itself. Much of this energy is then changed to heat energy,
which helps warm the Earth, keeping it in the temperature range that
living things have adapted to.
LIGHT AND ELECTROMAGNETIC RADIATION
Different
kinds of light are visible to different species. Humans see light in
what is called the visible range. It includes all the colors beginning
with red and continuing through orange, yellow, green, blue, and violet .
Some people can see farther into the violet region or the red region
than other people. Some animals have a different sensory range. Pit
vipers, for example, have sense organs (pits) that “see” rays that
humans feel as heat. These rays are called infrared radiation. Bees, on
the other hand, not only see some of the colors that humans see but are
also sensitive to ultraviolet radiation, which is beyond the range
visible to humans. So, though human eyes cannot detect them, infrared
rays and ultraviolet rays are related to visible light. Instruments
have been built that can detect and photograph objects by means of
infrared rays or ultraviolet rays. X rays, which can also be used to
photograph objects, are also related to light.
Scientists
have learned that all these forms of energy and many other kinds of
energy, such as radio waves, microwaves, and gamma rays, have the same
structure. They all consist of electrical and magnetic fields that work
together in a special way to form electromagnetic radiation.
SOURCES OF LIGHT
Unlike
many other animals, humans depend primarily on sight to learn about the
world around them. During the day early peoples could see by the light
that came from the sun; but night brought darkness and danger. One of
the most important steps people have taken to control their environment
occurred when they learned to conquer the dark by controlling fire—a
source of light.
Torches, candles, and oil lamps are all sources
of light. They depend on a chemical reaction—burning—to release the
energy we see as light. Plants and animals that glow in the
dark—glowworms, fireflies, and some mushrooms—change the chemical
energy stored in their tissues to light energy. Such creatures are
called bioluminescent. Electric-light bulbs and neon lights change
electrical energy, which may be produced by chemical, mechanical, or
atomic energy, into light energy.
Light sources are necessary for
vision. An object can be seen only if light travels from the object to
an eye that can sense it. When the object is itself a light source, it
is called luminous. Electric lights are luminous. The sun is a luminous
object because it is a source of light. An object that is not itself a
source of light must be illuminated by a luminous object before it can
be seen. The moon is illuminated by the sun. It is visible only where
the sun's rays hit it and bounce off toward Earth—or to an observer in
a spacecraft.
In a completely dark room, nothing is visible. When
a flashlight is turned on, its bulb and objects in its beam become
visible. If a bright overhead bulb is switched on, its light can bounce
off the walls, ceiling, floor, and furniture, making them and other
objects in its path visible.
Heating some things causes them to
give off visible light rays as well as invisible heat rays. This is the
case for electric-light filaments, red-hot burners on electric stoves,
and glowing coals. The light of such objects is incandescent. Other
light sources emit light energy but no heat energy. They are known as
luminescent, or cold light, sources. Neon and fluorescent lights are
luminescent.
MEASURING LIGHT
The
clarity with which an object can be seen depends in part on the amount
of light that falls on it, on how well it is illuminated. The amount of
light that a light source gives off (called its intensity) is one
factor in determining how well a surface will be illuminated by it.
Other factors are the slant of the illuminated surface in relation to
the light source and the distance between the surface and the source.
As a light beam travels outward from most light sources—the exceptions
include lasers and searchlights—the beam spreads to cover a larger
area. Distance greatly weakens illumination from such sources. The same
amount of light will cover a larger area if the surface it reaches is
moved farther away. This results in weaker illumination, following the
inverse-square law. If the distance from the source is doubled, the
amount of light falling on a given area is reduced to one fourth—the
inverse of two squared. If the distance is tripled, the area receives
only one ninth of the original illumination—the inverse of three
squared.
One way of varying the amount of illumination on a
surface is to vary the intensity of the light source. Intensity is
measured in candles (or candelas in the international system). A candle
used to be the amount of light given off by a carefully constructed wax
candle. It is now more precisely defined as one sixtieth of the light
intensity of one square centimeter of a perfectly black object at the
freezing point of platinum (2,046 K, or 3,223.4° F). Photometers are
devices that are used to measure the intensity of light sources.
People
are often more interested in measuring the illumination of a surface—a
desk top or the floor and walls of a room—than in measuring the light
that leaves the light fixture. When distance is measured in feet, the
illumination of a surface is measured in footcandles. At a distance of
one foot the illumination provided by a light source of 100 candles is
100 footcandles. Under the inverse-square law, the same source gives
one fourth as much illumination, or 25 footcandles, at a distance of
two feet.
Another measurement that scientists find useful is the
total amount of light energy that a source gives off over a certain
period of time. This amount of light energy is called the luminous flux
of a source and is measured in lumens. An ideal one-candle source gives
off 4 π lumens. One footcandle is equal to one lumen per square foot.
LIGHT AND MATTER
The
way substances look depends greatly on what happens when light hits
them. It is possible to see through transparent substances more or less
clearly because light can pass through them without being scattered or
stopped. Light that bounces off the objects behind a transparent
substance can pass right through it almost as if it were not in the
way. Clear window glass and clean water are transparent.
Only the
surfaces of opaque substances are visible. Light cannot pass through
them, and it is not possible to see through them. Opaque substances
either absorb or reflect light. The light energy they absorb usually
turns into heat and raises their temperature. Mercury, steel, and wood
are examples of opaque substances.
Translucent substances permit
some light to pass through them, but the light is scattered, and the
images of objects behind them are not retained. Usually, if translucent
substances are made thinner they become transparent; if they are made
thicker they become opaque. Frosted light bulbs, waxed paper, and some
kinds of curtain materials are translucent.
Reflection
Reflection
occurs when a light ray hits a surface and bounces off. The angle at
which the ray hits the surface is equal to the angle at which it
bounces off. If the surface is made very flat and smooth by polishing,
all the light rays bounce off in the same direction. This type of
reflection is called regular, specular, or mirror reflection. A mirror
surface forms an image of things that reflect light onto it. This
occurs because the light rays maintain the same pattern, except
reversed from left to right, that they possessed before being
reflected. Mirrors are usually made of smooth glass with a thin layer
of a shiny metal such as silver bonded to the rear side
When
an opaque surface is rough, even on the microscopic level, the light
rays that hit it are scattered, causing the surface itself to become
visible. This is diffuse, or irregular, reflection. If a piece of raw
steel with a rough opaque surface is polished smooth and flat, it
reflects light rays regularly and takes on the qualities of a mirror.
Refraction and Dispersion
Light
travels in a straight line as it passes through a transparent
substance. But when it moves from one transparent material to another
of different density—for example, from air to water or from glass to
air—it bends at the interface (where the two surfaces meet). This
bending is called refraction. The amount, or degree, of refraction is
related to the difference between the speeds of light in the two
materials of different densities—the greater the difference in
densities, the more the speed changes, and the greater the bend. A
slanting object partly out of water displays refraction. The object
appears to bend at the interface of the air and water. Lenses refract
light. Those that have concave, or hollowed-out, surfaces spread light
rays apart. Those that have convex, or bulging, surfaces bring light
rays closer together.
For centuries before the 1600s, scientists
had known that when a ray of white light shines on a prism, a broad
band containing several colors emerges. Some thought that the colors
were caused by variations in lightness and darkness. But in 1672 Isaac
Newton published the results of his experiments with light. He showed
that a second prism placed in the path of a beam of one color could not
add more color to the beam. It did, however, spread the beam farther
apart. Newton concluded that the first prism broke white light down
into its separate parts by spreading them apart, and he was able to
establish that white light is not a pure color but a combination of all
the colors in the spectrum.
A
prism spreads white light into the spectrum because each color has a
slightly different speed within the prism, so each color bends
(refracts) a slightly different amount as it enters and again as it
leaves the prism. Violet light slows up the most, so it is bent the
most; red light slows up the least, so it is bent the least. This
spreading apart of white light into a spectrum is called dispersion.
Physicists
often define dispersion as the fact that different colors move at
different speeds within a substance, not necessarily causing a
spectrum. For example, when white light enters a glass block that has
parallel faces, the colors all have different speeds and bend different
amounts as they travel through the glass. This is also dispersion. But
the colors all bend back to form white light as they leave the second
parallel face, so separate colors are not observed.
Opaque
materials absorb all the colors of white light except their own, which
they reflect. A piece of pure red material absorbs orange, yellow,
green, blue, and violet but reflects red. Transparent colored materials
absorb all colors except their own, which they both transmit and
reflect. A piece of pure blue cellophane absorbs red, orange, yellow,
green, and violet but transmits blue (it looks blue on the side
opposite the light source) and reflects blue (it looks blue on the same
side as the light source).
MEASURING THE SPEED OF LIGHT
Light
can travel through a vacuum. Stars are easily visible on clear nights,
though their light must travel for years through empty space before it
reaches Earth. A laboratory experiment demonstrates that light can
travel through a vacuum. When air is pumped out of a glass vacuum
chamber that contains a ringing bell, the bell remains visible while
the sound fades away. The vacuum cannot transmit sound waves, but the
light rays continue to pass through it.
It is much easier to
describe the interaction of light with matter than to explain what
light is. One reason for this is that light cannot be seen until it
interacts with matter—a beam of light is invisible unless it strikes an
eye or unless there are particles that reflect parts of the beam to an
eye. Also, light travels very fast—so fast that for centuries men
disputed whether it required any time for light to move from one point
to another.
Galileo suggested one of the first experiments to
measure the speed of light, and Italian scientists carried out his
idea. Two men were stationed on two hilltops. Each had a shaded
lantern. The first man was to uncover his lantern. As soon as the
second man saw the light, he was to uncover his lantern. The scientists
tried to measure the time that elapsed between the moment the first
lantern was uncovered and the moment a return beam was detected. The
speed of light was much too fast to be measured in this way, and the
scientists therefore concluded that light might well travel
instantaneously.
Olaus Roemer, a Danish astronomer, was dealing
with a different problem when he came across the first workable method
for measuring the speed of light. He was timing the eclipses of
Jupiter's moons and noticed that the time between eclipses varied by
several minutes. As the Earth approached Jupiter, the time between
eclipses grew shorter. As the Earth receded from Jupiter, the time
between eclipses grew longer. In 1676 Roemer proposed that these
discrepancies be used to calculate the time required for light to
travel the diameter of the Earth's orbit. Since the exact size of the
Earth's orbit was not yet known, and since Jupiter's irregular surface
caused errors in timing the eclipses, he did not arrive at an accurate
value for the speed of light. But he had demonstrated that light took
time to travel and that its speed was too quick to measure on Earth
with the instruments then available.
In 1849 Armand Fizeau, a
French physicist, devised a way to measure the speed of light on Earth
instead of relying on uncertain astronomical measurements. His
experimental apparatus included a beam of light that was sent through a
notch in a rotating disk, was reflected from a mirror, and returned to
the disk. The disk had 720 notches. When the returning light passed
through a notch, an observer could detect it; if it hit between
notches, the light was eclipsed. The distance light would travel (from
the open notch to the mirror and back to the point where a tooth could
eclipse the light) was measured. Fizeau timed the eclipses and observed
the rotational speed of the disk at the time of the eclipses. With this
information he calculated that the speed of light in air was 194,000
miles per second. Later investigators refined this method. Jean
Foucault replaced the disk with rotating mirrors and arrived at a value
of 186,000 miles per second.
One of the most surprising and
confusing facts about light was discovered by Albert Michelson and
Edward Morley. They measured the speed of light very accurately as it
traveled both in the same direction as the Earth's movement and in the
direction opposite to the Earth's movement. They expected to get
slightly different values, believing that the speed of the Earth would
be added to or subtracted from the speed of light. The situation, as
they saw it, was similar to that of a person looking out the window of
a car traveling at 60 miles per hour. If another car going 80 miles per
hour overtakes it, the second car then seems to be moving at a speed of
20 miles per hour, or its own speed
minus the speed of the car
it has passed. If a car going 80 miles per hour approaches a car going
60 miles per hour, it seems to be traveling at 140 miles per hour, or
its own speed
plus the speed of the car that it is approaching.
Light, the two men discovered, does not behave that way. Its speed
appears to be the same, no matter what the speed or direction of
movement of the observer making the measurement. Albert Einstein
developed his theory of relativity to help explain this phenomenon .
The accepted value for the speed of light in a vacuum is 2.997924562 × 10
8
meters per second (about 186,282 miles per second), a fundamental
constant of the universe. According to the theory of relativity, time
and distance may change as the speed of an object approaches the speed
of light (its length shrinks and any changes it regularly undergoes
take longer to occur, relative to a stationary observer), but the
measured value for the speed of light is constant.
LIGHT—WAVE OR PARTICLE?
By
the 17th century enough was known about the behavior of light for two
conflicting theories of its structure to emerge. One theory held that a
light ray was made up of a stream of tiny particles. The other regarded
light as a wave. Both of these views have been incorporated into the
modern theory of light.
Newton thought that light was composed of
tiny particles given off by light sources. He believed that the
different colors into which white light could be broken up were formed
by particles of different sizes. He thought refraction resulted from
the stronger attraction of the denser of two substances for the
particles of light. Since the attraction was greater, the speed of
light in denser mediums should also be greater, according to his
theory. A basic piece of evidence supporting the particle view of light
is that light travels in straight lines. This can be seen when a small,
steady light source shines on a relatively large object. The shadow of
the object has sharp borders. Newton felt that if light were a wave, it
would curve slightly around obstacles, giving fuzzy-edged shadows. He
pointed out that water waves curve as they pass an obstacle (for
example, dock pilings) and that sound waves curve over hills and around
the corners of buildings. Newton realized, however, that simple
variations in the size of particles did not explain all light
phenomena. When he tried to understand the shimmering coloration of
soap bubbles, he had to introduce the idea that the particles vibrated.
Christiaan
Huygens, a Dutch physicist, proposed that light was a wave. He
postulated that a substance called the ether (not to be confused with
the class of chemicals called ethers) filled the universe. Waves were
generated in this substance when light traveled through it. Huygens
assumed that light waves were like sound waves—the movement of
alternately compressed and rarefied ether. Such waves are called
longitudinal waves because the vibration of the wave is parallel to the
direction in which it is traveling.
Polarized Light
Neither
particle nor wave theory could really explain the polarization of light
by certain transparent crystals. Both Newton and Huygens knew that when
light was directed through certain crystals, it would emerge much
dimmer. If a second crystal of this class were placed at a certain
angle in the path of the dimmed light, the light could pass through it.
Then, as either of the two crystals was slowly turned, the light
emerging from the second crystal grew dimmer until it was completely
blocked. Evidently, something in the structure of the first crystal
allowed only part of the light to pass. When the second crystal was
lined up properly with the first, it allowed the same amount of light
to pass; when it was at the wrong angle to the first crystal, it
screened out the light from the first crystal.
Newton speculated
that polarization occurred because light particles had various shapes
on their sides, some of which were rejected by the crystal structure.
This was not a very satisfactory explanation. However, Huygens had to
make even more complicated assumptions to explain how crystals could
polarize longitudinal waves. Neither the wave theory nor the particle
theory was sufficiently developed to account for all the observed light
phenomena, but the weight of Newton's reputation caused the particle
theory to be accepted by most scientists.
Light Bends Around Corners
In
the early 19th century Thomas Young, a British physician, took the next
step in developing the wave theory. He demonstrated that light waves
were so short that the amount they curved as they passed an object was
too small to be visible. He showed that, though shadows from point
sources of light appear to have sharp edges, there are thin
light-and-dark bands along their borders that are caused by the bending
of some light rays into the shadow. This scattering of light, called
diffraction, can be observed under certain conditions. A thin tubular
source, such as a fluorescent light, is good. A very thin slit in an
opaque material, or even two fingers squeezed loosely together so that
light may pass between them, may cause diffraction. The slit is held a
foot or two in front of one eye, parallel to the light source; the
other eye is closed. Light shines through the slit, and a pattern of
colored bands, or a colored glow, can be seen outlining the slit. The
outline is colored because diffraction disperses white light into its
separate colors in much the same way that a prism does. Young observed
diffraction and concluded that it occurred because light was a wave.
Three
important measurements describe a wave—speed, frequency, and
wavelength. Frequency is a measure of the number of waves that pass a
given point in a specified amount of time. Wavelength is the distance
from one crest (the highest point) to the next crest, or from one
trough (the lowest point) to the next. If all the waves have the same
speed, a great many short waves will pass a point in the same time that
only a few long waves pass it. Speed equals the wavelength times the
frequency.
Young set up an experiment to measure the wavelength
of light; using the principle of interference. When two sets of waves
meet, they interfere with each other in a predictable way. Water waves,
such as those made by the wakes of two boats, illustrate this. When two
wakes meet, the water becomes choppy. Parts of the waves are very high
and parts are very low; the individual waves can reinforce each other
or cancel each other. Where two crests meet, the wave becomes higher.
Where two troughs meet, the wave becomes deeper. And if a crest and a
trough meet, they cancel each other and the water is level.
In
his interference experiment Young used a single light source, a pinhole
that admitted a single beam of sunlight. This beam fell on a screen
that had two pinholes close together. As light passed through each
pinhole, it curved and spread out (diffracted). Because the pinholes
were close enough together, the two light beams met and interfered with
each other. Their interference pattern was seen on a screen behind the
pinholes. With this pattern and knowing the distance between the
screens, Young was able to calculate that the wavelength of visible
light was about one millionth of a meter.
Subsequent measurements show that the wavelengths of visible light range from 7.60 × 10–5 centimeters to 3.85 × 10–5 centimeters (2.99 × 10–5 inches to 1.51 × 10–5
inches). Each color is associated with a range of wavelengths. Red has
the longest lengths. The wavelengths decrease from orange through
yellow, green, blue, and violet.
Transverse Waves Explain Polarization
Young
and Augustin Jean Fresnel, a French physicist, cooperated in developing
the idea that light waves are transverse, that they resemble the waves
made when a rope stretched from a post is jerked up and down rather
than longitudinal sound waves. The rope itself moves only up and down,
at right angles to the forward travel of the wave. Young and Fresnel
suggested the wave motion of light might also be at right angles to the
direction in which the wave was traveling. The motion could be in any
direction between sideways and up-and-down just so long as it was at
right angles to the direction of travel. Wave motion of this kind could
explain polarization. If a polarizing crystal admitted only those waves
that were vibrating in a certain direction, then a second crystal would
block those waves if it were turned at an angle to the first. The
second one would be oriented to accept only waves vibrating in a
different direction, and the first crystal would have already blocked
all those waves. Fresnel made calculations that accounted for all the
light behavior he knew of by assuming that light was made up of
transverse waves.
Measurements of the speed of light in
substances other than air presented additional difficulties for
Newton's particle theory of light. The theory had assumed that light
travels faster in dense substances than in rarefied substances. Fizeau
and Foucault measured the speed of light in various transparent
substances and discovered that it was slower in denser materials than
in air.
Invisible Light
Around
1800—while Young was developing his wave theory—three scientists
discovered that the color spectrum was bordered by invisible rays. Sir
William Herschel, a British astronomer, was measuring the temperature
of the colors dispersed by a prism. As he moved the thermometer down
the spectrum from violet to red, he observed a rise in temperature. As
he moved the thermometer beyond the red beam, the temperature grew even
higher. Herschel had discovered a hot, invisible radiation that
appeared to be a continuation of the spectrum. This radiation is called
infrared radiation because it occurs just below red in the spectrum,
where there is no visible light.
Ultraviolet rays were discovered
by Johann Wilhelm Ritter and by William Hyde Wollaston, who were
independently studying the effects of light on silver chloride. Silver
chloride placed in violet light grew dark. When the chemical was placed
in the area beyond the violet of the spectrum, it darkened even more
rapidly. They concluded that a chemically powerful kind of invisible
radiation lay beyond the violet end of the spectrum.
In 1864
James Clerk Maxwell, a Scottish physicist, published a theory of
electricity and magnetism. He had developed equations that predicted
the existence of electromagnetic waves caused by electrical
disturbances. He calculated the speed of such waves and found it to be
the same as the speed of light. Maxwell concluded that light was an
electromagnetic wave. As a single light wave travels through space, its
movement consists of the growth and collapse of electrical and magnetic
fields. The electrical fields are at right angles to the magnetic
fields, and both are at right angles to the direction in which the wave
is moving.
Maxwell's theory implied that other electromagnetic
radiations with wavelengths longer than infrared or shorter than
ultraviolet might be found. In 1887 Heinrich Hertz produced radio
waves, which have longer wavelengths than infrared rays, thus
confirming Maxwell's theory.
LIGHT: WAVE AND PARTICLE
In
1900 the German physicist Max Planck advanced a theory to account for
the behavior of blackbodies. A blackbody is an ideal substance with a
perfectly black surface that absorbs all the radiation that falls on it
and emits radiation in specific ways dependent on temperature. While
such an ideal material does not actually exist, some materials resemble
it closely enough to provide experimental tests of blackbody theory.
The observed behavior is that blackbodies do not emit all wavelengths
in equal amounts. Instead, certain wavelengths are emitted more often
than others. As the temperature increases, the wavelengths that are
emitted preferentially decrease in length. In other words, the
wavelength of maximum emission varies inversely with temperature.
Planck explained this behavior by suggesting that matter can handle
energy only in specific amounts, called quanta, and that amounts of
energy between these quanta cannot be absorbed or emitted.
In
1905 Einstein expanded this idea in his explanation of the
photoelectric effect. If light falls on certain metals, electrons in
those metals are freed and can form an electric current. Einstein was
trying to account for the observation that the energy of the electrons
is independent of the amount of radiation falling on the metal. The
maximum energy of the electrons was observed to depend on the
wavelength of the radiation. Einstein suggested that the photoelectric
effect could also be accounted for by assuming that electromagnetic
energy, including light, always occurs in these bundles. This
reintroduced the particle theory. The results of many subsequent
experiments supported the idea that light energy travels in quanta. An
individual light particle possesses one quantum of energy and is called
a photon.
The way matter becomes a light source can be explained
in terms of quantum theory. When certain elements are heated, they give
off light of a specific color. This light can be separated into a
spectrum that is made up of many distinct bright lines.
Each element has its unique spectrum, which can be accurately measured.
Since a spectrum positively identifies each element, the chemical
composition of astronomical bodies is determined by an analysis of
their spectra.
Scientists wondered why the atoms of each element,
when provided with a wide range of energies by the heating process,
give off only the specific energies in their spectra. The modern theory
of atomic structure makes this phenomenon understandable. An atom is
made up of a heavy, positively charged nucleus which is surrounded by
light, negatively charged electrons.
Modern theory states that
the electrons of an atom can assume certain fixed energy relationships,
called energy levels, to one another and to the nucleus. These energy
levels are the same for all the atoms of an element. An electron must
occupy one of the energy levels; it cannot possess any energy between
levels.
When an atom is heated, enough energy may be given to one
of the electrons to raise it to a higher energy level. But it usually
jumps back to a lower level, giving off an electromagnetic wave, which
is the energy difference of the two levels. When this energy is in the
visible light range, it shows up as one of the lines in the element's
visible spectrum. Each element has a different spectrum because each
element has a different number of electrons and different energy levels
available to these electrons.
In the early 20th century atomic
theory had not yet explained why both a wave theory and a particle
theory were needed to describe light. Physicists used both, depending
on which was more useful in a given situation. The paradox was finally
resolved in 1924 by Louis de Broglie. He postulated that matter, which
had always been treated as a collection of particles, had a wave aspect
as well. This wave nature has been demonstrated in experiments with
such particles as electrons.