Light
I INTRODUCTION
Light, form of energy visible to the human eye
that is radiated by moving charged particles. Light from the Sun provides the
energy needed for plant growth. Plants convert the energy in sunlight into
storable chemical form through a process called photosynthesis. Petroleum,
coal, and natural gas are the remains of plants that lived millions of years
ago, and the energy these fuels release when they burn is the chemical energy
converted from sunlight. When animals digest the plants and animals they eat,
they also release energy stored by photosynthesis.
Scientists have learned through experimentation that
light behaves like a particle at times and like a wave at other times. The
particle-like features are called photons. Photons are different from particles
of matter in that they have no mass and always move at the constant speed of
about 300,000 km/sec (186,000 mi/sec) when they are in a vacuum. When light
diffracts, or bends slightly as it passes around a corner, it shows wavelike behavior.
The waves associated with light are called electromagnetic waves because they
consist of changing electric and magnetic fields.
II THE NATURE OF LIGHT
To understand the nature of light and how it is normally
created, it is necessary to study matter at its atomic level. Atoms are the
building blocks of matter, and the motion of one of their constituents, the
electron, leads to the emission of light in most sources.
A Light Emission
Light can be emitted, or radiated, by electrons
circling the nucleus of their atom. Electrons can circle atoms only in certain
patterns called orbitals, and electrons have a specific amount of energy in
each orbital. The amount of energy needed for each orbital is called an energy
level of the atom. Electrons that circle close to the nucleus have less energy
than electrons in orbitals farther from the nucleus. If the electron is in the
lowest energy level, then no radiation occurs despite the motion of the
electron. If an electron in a lower energy level gains some energy, it must
jump to a higher level, and the atom is said to be excited. The motion of the
excited electron causes it to lose energy, and it falls back to a lower level.
The energy the electron releases is equal to the difference between the higher
and lower energy levels. The electron may emit this quantum of energy in the
form of a photon.
Each atom has a unique set of energy levels,
and the energies of the corresponding photons it can emit make up what is called
the atom’s spectrum. This spectrum is like a fingerprint by which the atom can
be identified. The process of identifying a substance from its spectrum is
called spectroscopy. The laws that describe the orbitals and energy levels of
atoms are the laws of quantum theory. They were invented in the 1920s
specifically to account for the radiation of light and the sizes of atoms.
B Electromagnetic Waves
The waves that accompany light are made up of
oscillating, or vibrating, electric and magnetic fields, which are force fields
that surround charged particles and influence other charged particles in their
vicinity. These electric and magnetic fields change strength and direction at
right angles, or perpendicularly, to each other in a plane (vertically and horizontally
for instance). The electromagnetic wave formed by these fields travels in a
direction perpendicular to the field’s strength (coming out of the plane). The
relationship between the fields and the wave formed can be understood by
imagining a wave in a taut rope. Grasping the rope and moving it up and down
simulates the action of a moving charge upon the electric field. It creates a
wave that travels along the rope in a direction that is perpendicular to the
initial up and down movement.
Because electromagnetic waves are transverse—that is, the
vibration that creates them is perpendicular to the direction in which they
travel, they are similar to waves on a rope or waves traveling on the surface
of water. Unlike these waves, however, which require a rope or water, light
does not need a medium, or substance, through which to travel. Light from the
Sun and distant stars reaches Earth by traveling through the vacuum of space.
The waves associated with natural sources of light
are irregular, like the water waves in a busy harbor. Scientists think of such
waves as being made up of many smooth waves, where the motion is regular and
the wave stretches out indefinitely with regularly spaced peaks and valleys.
Such regular waves are called monochromatic because they correspond to a single
color of light.
B1 Wavelength, Frequency, and Amplitude
The wavelength of a monochromatic wave is the
distance between two consecutive wave peaks. Wavelengths of visible light can
be measured in meters or in nanometers (nm), which are one-billionth of a meter
(or about 0.4 ten-millionths of an inch). Frequency corresponds to the number
of wavelengths that pass by a certain point in space in a given amount of time.
This value is usually measured in cycles per second, or hertz (Hz). All
electromagnetic waves travel at the same speed, so in one second, more short
waves will pass by a point in space than will long waves. This means that
shorter waves have a higher frequency than longer waves. The relationship
between wavelength, speed, and frequency is expressed by the equation: wave
speed equals wavelength times frequency, or
c = lf
Where c is the speed of a light
wave in m/sec (3x108 m/sec in a vacuum), l is the wavelength
in meters, and f is the wave’s frequency in Hz.
The amplitude of an electromagnetic wave is
the height of the wave, measured from a point midway between a peak and a
trough to the peak of the wave. This height corresponds to the maximum strength
of the electric and magnetic fields and to the number of photons in the light.
B2 Electromagnetic Spectrum
The electromagnetic spectrum refers to the entire range
of frequencies or wavelengths of electromagnetic waves (see Electromagnetic
Radiation). Light traditionally refers to the range of frequencies that can be seen
by humans. The frequencies of these waves are very high, about one-half to
three-quarters of a million billion (5 x 1014 to 7.5 x 1014)
Hz. Their wavelengths range from 400 to 700 nm. X rays have wavelengths ranging
from several thousandths of a nanometer to several nanometers, and radio waves
have wavelengths ranging from several meters to several thousand meters.
Waves with frequencies a little lower than the range of
human vision (and with wavelengths correspondingly longer) are called infrared.
Waves with frequencies a little higher and wavelengths shorter than human eyes
can see are called ultraviolet. About half the energy of sunlight at Earth’s
surface is visible electromagnetic waves, about 3 percent is ultraviolet, and
the rest is infrared.
Each different frequency or wavelength of visible light
causes our eye to see a slightly different color. The longest wavelength we can
see is deep red at about 700 nm. The shortest wavelength humans can detect is
deep blue or violet at about 400 nm. Most light sources do not radiate
monochromatic light. What we call white light, such as light from the Sun, is a
mixture of all the colors in the visible spectrum, with some represented more
strongly than others. Human eyes respond best to green light at 550 nm, which
is also approximately the brightest color in sunlight at Earth’s surface.
B3 Polarization
Polarization refers to the direction of the electric
field in an electromagnetic wave. A wave whose electric field is oscillating in
the vertical direction is said to be polarized in the vertical direction. The
photons of such a wave would interact with matter differently than the photons
of a wave polarized in the horizontal direction. The electric field in light
waves from the Sun vibrates in all directions, so direct sunlight is called
unpolarized. Sunlight reflected from a surface is partially polarized parallel
to the surface. Polaroid sunglasses block light that is horizontally polarized
and therefore reduce glare from sunlight reflecting off horizontal surfaces.
C Photons
Photons may be described as packets of light
energy, and scientists use this concept to refer to the particle-like aspect of
light. Photons are unlike conventional particles, such as specks of dust or
marbles, however, in that they are not limited to a specific volume in space or
time. Photons are always associated with an electromagnetic wave of a definite
frequency. In 1900 the German physicist Max Planck discovered that light energy
is carried by photons. He found that the energy of a photon is equal to the
frequency of its electromagnetic wave multiplied by a constant called h,
or Planck's constant. This constant is very small because one photon carries
little energy. Using the watt-second, or joule, as the unit of energy, Planck’s
constant is 6.626 x 10-34 (a decimal point followed by 33 zeros and
then the number 6626) joule-seconds in exponential notation. The energy
consumed by a one-watt light bulb in one second, for example, is equivalent to
two and a half million trillion photons of green light. Sunlight warms one
square meter at the top of Earth’s atmosphere at noon at the equator with the
equivalent of about 14 100-watt light bulbs. Light waves from the Sun,
therefore, produce a very large number of photons.
D Sources of Light
Sources of light differ in how they provide energy
to the charged particles, such as electrons, whose motion creates the light. If
the energy comes from heat, then the source is called incandescent. If the
energy comes from another source, such as chemical or electric energy, the
source is called luminescent (see Luminescence).
D1 Incandescence
In an incandescent light source, hot atoms collide
with one another. These collisions transfer energy to some electrons, boosting
them into higher energy levels. As the electrons release this energy, they emit
photons. Some collisions are weak and some are strong, so the electrons are
excited to different energy levels and photons of different energies are
emitted. Candle light is incandescent and results from the excited atoms of
soot in the hot flame. Light from an incandescent light bulb comes from excited
atoms in a thin wire called a filament that is heated by passing an electric
current through it.
The Sun is an incandescent light source, and
its heat comes from nuclear reactions deep below its surface. As the nuclei of
atoms interact and combine in a process called nuclear fusion, they release
huge amounts of energy. This energy passes from atom to atom until it reaches
the surface of the Sun, where the temperature is about 6000°C (11,000°F).
Different stars emit incandescent light of different frequencies—and therefore
color—depending on their mass and their age.
All thermal, or heat, sources have a broad
spectrum, which means they emit photons with a wide range of energies. The
color of incandescent sources is related to their temperature, with hotter
sources having more blue in their spectra, or ranges of photon energies, and
cooler sources more red. About 75 percent of the radiation from an incandescent
light bulb is infrared. Scientists learn about the properties of real
incandescent light sources by comparing them to a theoretical incandescent
light source called a black body. A black body is an ideal incandescent light
source, with an emission spectrum that does not depend on what material the
light comes from, but only its temperature.
D2 Luminescence
A luminescent light source absorbs energy in some
form other than heat, and is therefore usually cooler than an incandescent
source. The color of a luminescent source is not related to its temperature. A
fluorescent light is a type of luminescent source that makes use of chemical
compounds called phosphors. Fluorescent light tubes are filled with mercury
vapor and coated on the inside with phosphors. As electricity passes through
the tube, it excites the mercury atoms and makes them emit blue, green, violet,
and ultraviolet light. The electrons in phosphor atoms absorb the ultraviolet
radiation, then release some energy to heat before
emitting visible light with a lower frequency.
Phosphor compounds are also used to convert electron
energy to light in a television picture tube. Beams of electrons in the tube
collide with phosphor atoms in small dots on the screen, exciting the phosphor
electrons to higher energy levels. As the electrons drop back to their original
energy level, they emit some heat and visible light. The light from all the
phosphor dots combines to form the picture.
In certain phosphor compounds, atoms remain excited
for a long time before radiating light. A light source is called phosphorescent
if the delay between energy absorption and emission is longer than one second.
Phosphorescent materials can glow in the dark for several minutes after they
have been exposed to strong light.
The aurora borealis and aurora australis
(northern and southern lights) in the night sky in high latitudes are
luminescent sources. Electrons in the solar wind that sweeps out from the Sun
become deflected in Earth’s magnetic field and dip into the upper atmosphere near
the north and south magnetic poles. The electrons then collide with atmospheric
molecules, exciting the molecules’ electrons and making them emit light in the
sky.
Chemiluminescence occurs when a chemical reaction produces
molecules with electrons in excited energy levels that can then radiate light.
The color of the light depends on the chemical reaction. When chemiluminescence
occurs in plants or animals it is called bioluminescence. Many creatures, from
bacteria to fish, make light this way by manufacturing substances called
luciferase and luciferin. Luciferase helps luciferin combine with oxygen, and
the resulting reaction creates excited molecules that emit light. Fireflies use
flashes of light to attract mates, and some fish use bioluminescence to attract
prey, or confuse predators.
D3 Synchrotron Radiation
Not all light comes from atoms. In a
synchrotron light source, electrons are accelerated by microwaves and kept in a
circular orbit by large magnets. The whole machine, called a synchrotron, resembles
a large artificial atom. The circulating electrons can be made to radiate very
monochromatic light at a wide range of frequencies.
D4 Lasers
A laser is a special kind of light source
that produces very regular waves that permit the light to be very tightly
focused. Laser is actually an acronym for Light Amplification
by Stimulated Emission of Radiation. Each radiating charge
in a nonlaser light source produces a light wave that may be a little different
from the waves produced by the other charges. Laser sources have atoms whose
electrons radiate all in step, or synchronously. As a result, the electrons
produce light that is polarized, monochromatic, and coherent, which means that
its waves remain in step, with their peaks and troughs coinciding, over long
distances.
This coherence is made possible by the phenomenon
of stimulated emission. If an atom is immersed in a light wave with a
frequency, polarization, and direction the same as light that the atom could
emit, then the radiation already present stimulates the atom to emit more of
the same, rather than emit a slightly different wave. So the existing light is
amplified by the addition of one more photon from the atom. A luminescent light
source can provide the initial amplification, and mirrors are used to continue
the amplification.
Lasers have many applications in medicine,
scientific research, military technology, and communications. They provide a
very focused, powerful, and controllable energy source that can be used to
perform delicate tasks. Laser light can be used to drill holes in diamonds and
to make microelectronic components. The precision of lasers helps doctors
perform surgery without damaging the surrounding tissue. Lasers are useful for
space communications because laser light can carry a great deal of information
and travel long distances without losing signal strength.
E Detection of Light
For each way of producing light there is a
corresponding way of detecting it. Just as heat produces incandescent light,
for example, light produces measurable heat when it is absorbed by a material.
E1 Photoelectric Effect
The photoelectric effect is a process in which an atom
absorbs a photon that has so much energy that the photon sets one of the atom’s
electrons free to move outside the atom. Part of the photon’s energy goes
toward releasing the electron from the atom. This energy is called the
activation energy of the electron. The rest of the photon’s energy is transferred
to the released electron in the form of motion, or kinetic energy. Since the
photon energy is proportional to frequency, the released electron, or
photoelectron, moves faster when it has absorbed high-frequency light.
Metals with low activation energies are used to
make photodetectors and photoelectric cells whose electrical properties change
in the presence of light. Solar cells use the photoelectric effect to convert
sunlight into electricity. Solar cells are used in place of electric batteries
in remote applications like space satellites or roadside emergency telephones (see
Solar Energy). Hand-held calculators and watches often use solar cells so
that battery replacement is unnecessary.
E2 Photochemical Detection
The change induced in photographic film exposed to
light is an example of photochemical detection of photons. Light induces a
chemical change in photosensitive chemicals on film. The film is then processed
to convert the chemical change into a permanent image and to remove the
photosensitive chemicals from the film so it will not continue to change when
it is viewed in full light.
Human vision works on a similar principle. Light of
different frequencies causes different chemical changes in the eye. The
chemical action generates nerve impulses that our brains interpret as color,
shape, and location of objects.
III BEHAVIOR OF LIGHT
Light behavior can be divided into two categories:
how light interacts with matter and how light travels, or propagates through
space or through transparent materials. The propagation of light has much in
common with the propagation of other kinds of waves, including sound waves and
water waves.
A Interaction with Material
When light strikes a material, it interacts with
the atoms in the material, and the corresponding effects depend on the
frequency of the light and the atomic structure of the material. In transparent
materials, the electrons in the material oscillate, or vibrate, while the light
is present. This oscillation momentarily takes energy away from the light and
then puts it back again. The result is to slow down the light wave without
leaving energy behind. Denser materials generally slow the light more than less
dense materials, but the effect also depends on the frequency or wavelength of
the light. Under certain laboratory conditions, scientists can slow light down.
In 2001 scientists brought a beam of light to a halt by temporarily trapping it
within an extremely cold cloud of sodium atoms.
Materials that are not completely transparent
either absorb light or reflect it. In absorbing materials, such as dark colored
cloth, the energy of the oscillating electrons does not go back to the light.
The energy instead goes toward increasing the motion of the atoms, which causes
the material to heat up. The atoms in reflective materials, such as metals,
re-radiate light that cancels out the original wave. Only the light re-radiated
back out of the material is observed. All materials exhibit some degree of
absorption, refraction, and reflection of light. The study of the behavior of
light in materials and how to use this behavior to control light is called
optics.
A1 Refraction
Refraction is the bending of light when it passes
from one kind of material into another. Because light travels at a different
speed in different materials, it must change speeds at the boundary between two
materials. If a beam of light hits this boundary at an angle, then light on the
side of the beam that hits first will be forced to slow down or speed up before
light on the other side hits the new material. This makes the beam bend, or
refract, at the boundary. Light bouncing off an object underwater, for
instance, travels first through the water and then through the air to reach an
observer’s eye. From certain angles an object that is partially submerged
appears bent where it enters the water because light from the part underwater
is being refracted.
The refractive index of a material is the ratio of
the speed of light in a vacuum to the speed of light inside the material.
Because light of different frequencies travels at different speeds in a
material, the refractive index is different for different frequencies. This
means that light of different colors is bent by different angles as it passes
from one material into another. This effect produces the familiar colorful
spectrum seen when sunlight passes through a glass prism. The angle of bending
at a boundary between two transparent materials is related to the refractive
indexes of the materials through Snell’s Law, a mathematical formula that is
used to design lenses and other optical devices to control light.
A2 Reflection
Reflection also occurs when light hits the boundary
between two materials. Some of the light hitting the boundary will be reflected
into the first material. If light strikes the boundary at an angle, the light
is reflected at the same angle, similar to the way balls bounce when they hit
the floor. Light that is reflected from a flat boundary, such as the boundary
between air and a smooth lake, will form a mirror image. Light reflected from a
curved surface may be focused into a point, a line, or onto an area, depending
on the curvature of the surface.
A3 Scattering
Scattering occurs when the atoms of a transparent material
are not smoothly distributed over distances greater than the length of a light
wave, but are bunched up into lumps of molecules or particles. The sky is
bright because molecules and particles in the air scatter sunlight. Light with
higher frequencies and shorter wavelengths is scattered more than light with
lower frequencies and longer wavelengths. The atmosphere scatters violet light
the most, but human eyes do not see this color, or frequency, well. The eye
responds well to blue, though, which is the next most scattered color. Sunsets
look red because when the Sun is at the horizon, sunlight has to travel through
a longer distance of atmosphere to reach the eye. The thick layer of air, dust
and haze scatters away much of the blue. The spectrum of light scattered from
small impurities within materials carries important information about the
impurities. Scientists measure light scattered by the
atmospheres of other planets in the solar system to learn about the chemical
composition of the atmospheres.
B How Light Travels
The first successful theory of light wave motion in
three dimensions was proposed by Dutch scientist Christiaan Huygens in 1678.
Huygens suggested that light wave peaks form surfaces like the layers of an
onion. In a vacuum, or a uniform material, the surfaces are spherical. These
wave surfaces advance, or spread out, through space at the speed of light.
Huygens also suggested that each point on a wave surface can act like a new
source of smaller spherical waves, which may be called wavelets,
that are in step with the wave at that point. The envelope of all the
wavelets is a wave surface. An envelope is a curve or surface that touches a
whole family of other curves or surfaces like the wavelets. This construction
explains how light seems to spread away from a pinhole rather than going in one
straight line through the hole. The same effect blurs the edges of shadows.
Huygens’s principle, with minor modifications, accurately describes all forms
of wave motion.
B1 Interference
Interference in waves occurs when two waves overlap. If
a peak of one wave is aligned with the peak of the second wave, then the two
waves will produce a larger wave with a peak that is the sum of the two
overlapping peaks. This is called constructive interference. If a peak of one
wave is aligned with a trough of the other, then the waves will tend to cancel
each other out and they will produce a smaller wave or no wave at all. This is
called destructive interference.
In 1803 English scientist Thomas Young studied
interference of light waves by letting light pass through a screen with two
slits. In this configuration, the light from each slit spreads out according to
Huygens’s principle and eventually overlaps with light from the other slit. If
a screen is set up in the region where the two waves overlap, a point on the
screen will be light or dark depending on whether the two waves interfere
constructively or destructively. If the difference between the distance from
one slit to a point on the screen and the other slit to the same point on the
screen is an exact number of wavelengths, then light waves arriving at that
point will be in step and constructively interfere, making the point bright. If
the difference is an exact odd number of half wavelengths, then light waves will
arrive out of step, with one wave’s trough arriving at the same time as another
wave’s peak. The waves will destructively interfere, making the point dark. The
resulting pattern is a series of parallel bright and dark lines on the screen.
Instruments called interferometers use various
arrangements of reflectors to produce two beams of light, which are allowed to
interfere. These instruments can be used to measure tiny differences in
distance or in the speed of light in one of the beams by observing the interference
pattern produced by the two beams.
Holography is another application of interference. A
hologram is made by splitting a light wave in two with a partially reflecting
mirror. One part of the light wave travels through the mirror and is sent directly
to a photographic plate. The other part of the wave is reflected first toward a
subject, a face for example, and then toward the plate. The resulting
photograph is a hologram. Instead of being an image of the face, it is an image
of the interference pattern between the two beams. A normal photograph records
only the light and dark features of the face and ignores the positions of peaks
and troughs of the light wave that form the interference pattern. Since the
full light wave is restored when a hologram is illuminated, the viewer can see
whatever the original wave contained, including the three-dimensional quality
of the original face.
B2 Diffraction
Diffraction is the spreading of light waves as they
pass through a small opening or around a boundary. Young’s principle of
interference can be applied to Huygens’s explanation of diffraction to explain
fringe patterns in diffracted light. As a beam of light emerges from a slit in
an illuminated screen, the light some distance away from the screen will
consist of overlapping wavelets from different points of the light wave in the
opening of the slit. When the light strikes a spot on a display screen across
from the slit, these points are at different distances from the spot, so their
wavelets can interfere and lead to a pattern of light and dark regions. The
pattern produced by light from a single slit will not be as pronounced as a
pattern from two slits. This is because there are an infinite number of interfering
waves, one from each point emerging from the slit, and their interference
patterns overlap one another.
IV MEASURING LIGHT
Monochromatic light, or light of one color, has several
characteristics that can be measured. As discussed in the section on
electromagnetic waves, the length of light waves is measured in meters, and the
frequency of light waves is measured in hertz. The wavelength can be measured
with interferometers, and the frequency determined from the wavelength and a
measurement of the velocity of light in meters per second. Monochromatic light
also has a well-defined polarization that can be measured using devices called
polarimeters. Sometimes the direction of scattered light is also an important
quantity to measure.
When light is considered as a source of
illumination for human eyes, its intensity, or brightness, is measured in units
that are based on a modernized version of the perceived brightness of a candle.
These units include the rate of energy flow in light, which, for monochromatic
light traveling in a single direction, is determined by the rate of flow of
photons. The rate of energy flow in this case can be stated in watts, or Joules per second. Usually light contains many
colors and radiates in many directions away from a source such as a lamp.
A Brightness
Scientists use the units
candela and lumen to measure the brightness of light as perceived by humans.
These units account for the different response of the eye to light of different
colors. The lumen measures the total amount of energy in the light radiated in
all directions, and the candela measures the amount radiated in a particular
direction. The candela was originally called the candle, and it was
defined in terms of the light produced by a standard candle. It is now defined
as the energy flow in a given direction of a yellow-green light with a
frequency of 540 x 1012 Hz and a radiant intensity, or energy
output, of 1/683 watt into the opening of a cone of one steradian. The
steradian is a measure of angle in three dimensions.
The lumen can be defined in terms of a source
that radiates one candela uniformly in all directions. If a sphere with a
radius of one foot were centered on the light source, then one square foot of
the inside surface of the sphere would be illuminated with a flux of one lumen.
Flux means the rate at which light energy is falling on the surface. The
illumination, or luminance, of that one square foot is defined to be one
foot-candle.
The illumination at a different distance from a
source can be calculated from the inverse square law: One lumen of flux spreads
out over an area that increases as the square of the distance from the center
of the source. This means that the light per square foot decreases as the
inverse square of the distance from the source. For instance, if 1 square foot
of a surface that is 1 foot away from a source has an illumination of 1
foot-candle, then 1 square foot of a surface that is 4 feet away will have an
illumination of 1/16 foot-candle. This is because 4 feet away from the source,
the 1 lumen of flux landing on 1 square foot has had to spread out over 16
square feet. In the metric system, the unit of luminous flux is also called the
lumen, and the unit of illumination is defined in meters and is called the lux.
B The Speed of Light
Scientists have defined the speed of light in a
vacuum to be exactly 299,792,458 meters per second (about 186,000 miles per
second). This definition is possible because since 1983, scientists have known
the distance light travels in one second more accurately than the definition of
the standard meter. Therefore, in 1983, scientists defined the meter as
1/299,792,458, the distance light travels through a vacuum in one second. This
precise measurement is the latest step in a long history of measurement,
beginning in the early 1600s with an unsuccessful attempt by Italian scientist
Galileo to measure the speed of lantern light from one hilltop to another.
The first successful measurements of the speed of
light were astronomical. In 1676 Danish astronomer Olaus Roemer noticed a delay
in the eclipse of a moon of Jupiter when it was viewed from the far side as
compared with the near side of Earth’s orbit. Assuming the delay was the travel
time of light across Earth’s orbit, and knowing roughly the orbital size from
other observations, he divided distance by time to estimate the speed.
English physicist James Bradley obtained a better
measurement in 1729. Bradley found it necessary to keep changing the tilt of
his telescope to catch the light from stars as Earth went around the Sun. He
concluded that Earth’s motion was sweeping the telescope sideways relative to
the light that was coming down the telescope. The angle of tilt, called the
stellar aberration, is approximately the ratio of the orbital speed of Earth to
the speed of light. (This is one of the ways scientists determined that Earth
moves around the Sun and not vice versa.)
In the mid-19th century, French physicist Armand
Fizeau directly measured the speed of light by sending a narrow beam of light
between gear teeth in the edge of a rotating wheel. The beam then traveled a
long distance to a mirror and came back to the wheel where, if the spin were
fast enough, a tooth would block the light. Knowing the distance to the mirror
and the speed of the wheel, Fizeau could calculate the speed of light. During
the same period, the French physicist Jean Foucault made other, more accurate
experiments of this sort with spinning mirrors.
Scientists needed accurate measurements of the speed of
light because they were looking for the medium that light traveled in. They
called the medium ether, which they believed waved to produce the light. If
ether existed, then the speed of light should appear larger or smaller
depending on whether the person measuring it was moving toward or away from the
ether waves. However, all measurements of the speed of light in different
moving reference frames gave the same value.
In 1887 American physicists Albert A. Michelson and
Edward Morley performed a very sensitive experiment designed to detect the
effects of ether. They constructed an interferometer with two light beams—one
that pointed along the direction of Earth’s motion, and one that pointed in a
direction perpendicular to Earth’s motion. The beams were reflected by mirrors
at the ends of their paths and returned to a common point where they could
interfere. Along the first beam, the scientists expected Earth’s motion to
increase or decrease the beam’s velocity so that the number of wave cycles
throughout the path would be changed slightly relative to the second beam,
resulting in a characteristic interference pattern. Knowing the velocity of
Earth, it was possible to predict the change in the number of cycles and the
resulting interference pattern that would be observed. The Michelson-Morley
apparatus was fully capable of measuring it, but the scientists did not find
the expected results.
The paradox of the constancy of the speed of
light created a major problem for physical theory that German-born American physicist
Albert Einstein finally resolved in 1905. Einstein suggested that physical
theories should not depend on the state of motion of the observer. Instead,
Einstein said the speed of light had to remain constant, and all the rest of
physics had to be changed to be consistent with this fact. This special theory
of relativity predicted many unexpected physical consequences, all of which
have since been observed in nature.
V HISTORY OF LIGHT THEORIES
The earliest speculations about light were hindered by
the lack of knowledge about how the eye works. The Greek philosophers from as
early as Pythagoras, who lived during the 5th century bc, believed light
issued forth from visible things, but most also thought vision, as distinct
from light, proceeded outward from the eye. Plato gave a version of this theory
in his dialogue Timaeus, written in the 3rd century bc, which greatly
influenced later thought.
Some early ideas of the Greeks, however, were
correct. The philosopher and statesman Empedocles believed that light travels
with finite speed, and the philosopher and scientist Aristotle accurately
explained the rainbow as a kind of reflection from raindrops. The Greek
mathematician Euclid understood the law of reflection and the properties of
mirrors. Early thinkers also observed and recorded the phenomenon of
refraction, but they did not know its mathematical law. The mathematician and
astronomer Ptolemy was the first person on record to collect experimental data
on optics, but he too believed vision issued from the eye. His work was further
developed by Egyptian scientist Ibn al Haythen, who worked in Iraq and Egypt
and was known to Europeans as Alhazen. Through logic and experimentation,
Alhazen finally discounted Plato’s theory that vision issued forth from the
eye. In Europe, Alhazen was the most well known among a group of Islamic
scholars who preserved and built upon the classical Greek tradition. His work
influenced all later investigations on light.
A Early Scientific Theories
The early modern scientists Galileo, Johannes
Kepler of Germany, and René Descartes of France all made contributions to the
understanding of light. Descartes discussed optics and reported the law of
refraction in his famous Discours de la méthode (Discourse on Method),
published in 1637. The Dutch astronomer and mathematician Willebrord Snell
independently discovered the law of refraction in 1620, and the law is now
named after him.
During the late 1600s, an important question
emerged: Is light a swarm of particles or is it a wave in some pervasive medium
through which ordinary matter freely moves? English physicist Sir Isaac Newton
was a proponent of the particle theory, and Huygens developed the wave theory
at about the same time. At the time it seemed that wave theories could not
explain optical polarization because waves that scientists were familiar with
moved parallel, not perpendicular, to the direction of wave travel. On the
other hand, Newton had difficulty explaining the phenomenon of interference of
light. His explanation forced a wavelike property on a particle description.
Newton’s great prestige coupled with the difficulty of explaining polarization
caused the scientific community to favor the particle theory, even after
English physicist Thomas Young analyzed a new class of interference phenomena
using the wave theory in 1803.
The wave theory was finally accepted after
French physicist Augustin Fresnel supported Young’s ideas with mathematical
calculations in 1815 and predicted surprising new effects. Irish mathematician
Sir William Hamilton clarified the relationship between wave and particle
viewpoints by developing a theory that unified optics and mechanics. Hamilton’s
theory was important in the later development of quantum mechanics.
Between the time of Newton and Fresnel,
scientists developed mathematical techniques to describe wave phenomena in
fluids and solids. Fresnel and his successors were able to use these advances
to create a theory of transverse waves that would account for the phenomenon of
optical polarization. As a result, an entire wave theory of light existed in
mathematical form before British physicist James Clerk Maxwell began his work
on electromagnetism. In his theory of electromagnetism, Maxwell showed that
electric and magnetic fields affect each other in such a way as to permit waves
to travel through space. The equations he derived to describe these
electromagnetic waves matched the equations scientists already knew to describe
light. Maxwell’s equations, however, were more general in that they described
electromagnetic phenomena other than light and they predicted waves throughout
the electromagnetic spectrum. In addition, his theory gave the correct speed of
light in terms of the properties of electricity and magnetism. When German
physicist Gustav Hertz later detected electromagnetic waves at lower
frequencies, which the theory predicted, the basic correctness of Maxwell’s
theory was confirmed.
Maxwell’s work left unsolved a problem common to
all wave theories of light. A wave is a continuous phenomenon, which means that
when it travels, its electromagnetic field must move at each of the infinite
number of points in every small part of space. When we add heat to any system
to raise its temperature, the energy is shared equally among all the parts of
the system that can move. When this idea is applied to light, with an infinite
number of moving parts, it appears to require an infinite amount of heat to
give all the parts equal energy. But thermal radiation, the process in which
heated objects emit electromagnetic waves, occurs in nature with a finite
amount of heat. Something that could account for this process was missing from
Maxwell’s theory. In 1900 Max Planck provided the missing concept. He proposed
the existence of a light quantum, a finite packet of energy that became known
as the photon.
B Modern Theory
Planck’s theory remained mystifying until Einstein
showed how it could be used to explain the photoelectric effect, in which the
speed of ejected electrons was related not to the intensity of light but to its
frequency. This relationship was consistent with Planck’s theory, which
suggested that a photon’s energy was related to its frequency. During the next
two decades scientists recast all of physics to be consistent with Planck’s
theory. The result was a picture of the physical world that was different from
anything ever before imagined. Its essential feature is that all matter appears
in physical measurements to be made of quantum bits, which are something like
particles. Unlike the particles of Newtonian physics, however, a quantum
particle cannot be viewed as having a definite path of movement that can be
predicted through laws of motion. Quantum physics only permits the prediction
of the probability of where particles may be found. The probability is the
squared amplitude of a wave field, sometimes called the wave function
associated with the particle. For photons the underlying probability field is
what we know as the electromagnetic field. The current world view that
scientists use, called the Standard Model, divides particles into two
categories: fermions (building blocks of atoms, such as electrons,
protons, and neutrons), which cannot exist in the same place at the same time,
and bosons, such as photons, which can (see Elementary Particles).
Bosons are the quantum particles associated with the force fields that act on
the fermions. Just as the electromagnetic field is a combination of electric
and magnetic force fields, there is an even more general field called the
electroweak field. This field combines electromagnetic forces and the weak
nuclear force. The photon is one of four bosons associated with this field. The
other three bosons have large masses and decay, or break apart, quickly to
lighter components outside the nucleus of the atom.
Reviewed By: John H. Marburger
Microsoft ® Encarta ® Reference Library 2003. ©
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