The exact nature of matter and visible light is a mystery that has puzzled man for centuries. Greek scientists from the ancient Pythagorean discipline postulated that every visible object emits a steady stream of particles, while Aristotle concluded that light travels in a manner similar to waves in the ocean. Even though these ideas have undergone numerous modifications and a significant degree of evolution over the past 20 centuries, the essence of the dispute established by the Greek philosophers remains to this day.
One
point of view envisions light as wave-like in nature,
producing energy that traverses through space in a
manner similar to the ripples spreading across the
surface of a still pond after being disturbed by a
dropped rock. The opposing view holds that light is
composed of a steady stream of particles, much like tiny
droplets of water sprayed from a garden hose nozzle.
During the past few centuries, the consensus of opinion has wavered with one view prevailing for a period of time, only to be overturned by evidence for the other. Only during the first decades of the twentieth century was enough compelling evidence collected to provide a comprehensive answer, and to everyone's surprise, both theories turned out to be correct, at least in part.
In the early eighteenth century, the argument about the nature of light had turned the scientific community into divided camps that fought vigorously over the validity of their favorite theories. One group of scientists, who subscribed to the wave theory, centered their arguments on the discoveries of Dutchman Christiaan Huygens. The opposing camp cited Sir Isaac Newton's prism experiments as proof that light traveled as a shower of particles, each proceeding in a straight line until it was refracted, absorbed, reflected, diffracted or disturbed in some other manner. Although Newton, himself, appeared to have some doubt about his corpuscular theory on the nature of light, his prestige in the scientific community held so much weight that his advocates ignored all other evidence during their ferocious battles.
Huygens'
theory of light refraction, based on the concept of the
wave-like nature of light, held that the velocity of
light in any substance was inversely proportion to its
refractive index. In other words, Huygens postulated
that the more light was "bent" or refracted by a
substance, the slower it would move while traversing
across that substance. His followers concluded that if
light were composed of a stream of particles, then the
opposite effect would occur because light entering a
denser medium would be attracted by molecules in the
medium and experience an increase, rather than a
decrease, in speed. Although the perfect solution to
this argument would be to measure the speed of light in
different substances, air and glass for example, the
devices of the period were not up to the task. Light
appeared to move at the same speed regardless of the
material through which it passed. Over 150 years passed
before the speed of light could be measured with a high
enough accuracy to prove that the Huygens theory was
correct.
Despite the highly regarded reputation of Sir Isaac Newton, a number of prominent scientists in the early 1700s did not agree with his corpuscular theory. Some argued that if light consisted of particles, then when two beams are crossed, some of the particles would collide with each other to produce a deviation in the light beams. Obviously, this is not the case, so they concluded that light must not be composed of individual particles.
Particle and Wave Refraction
When a beam of light travels between two media having differing refractive indices, the beam undergoes refraction, and changes direction when it passes from the first medium into the second. This interactive tutorial explores how particles and waves behave when refracted through a transparent surface.
Huygens, for all his intuition, had suggested in his 1690 treatise Traité de la Lumière that light waves traveled through space mediated by the ether, a mystical weightless substance, which exists as an invisible entity throughout air and space. The search for ether consumed a significant amount of resources during the nineteenth century before finally being laid to rest. The ether theory lasted at least until the late 1800s, as evidenced by Charles Wheatstone's proposed model demonstrating that ether carried light waves by vibrating at an angle perpendicular to the direction of light propagation, and James Clerk Maxwell's detailed models describing the construction of the invisible substance. Huygens believed that ether vibrated in the same direction as light, and formed a wave itself as it carried the light waves. In a later volume, Huygens' Principle, he ingeniously described how each point on a wave could produce its own wavelets, which then add together to form a wavefront. Huygens employed this idea to produce a detailed theory for the refraction phenomenon, and also to explain why light rays do not crash into each other when they cross paths.
When
a beam of light travels between two media having
different refractive indices, the beam undergoes
refraction, and changes direction when it passes from
the first medium into the second. To determine whether
the light beam is composed of waves or particles, a
model for each can be devised to explain the phenomenon
(Figure 3). According to Huygens' wave theory, a small
portion of each angled wavefront should impact the
second medium before the rest of the front reaches the
interface. This portion will start to move through the
second medium while the rest of the wave is still
traveling in the first medium, but will move more slowly
due to the higher refractive index of the second medium.
Because the wavefront is now traveling at two different
speeds, it will bend into the second medium, thus
changing the angle of propagation. In contrast, particle
theory has a rather difficult time explaining why
particles of light should change direction when they
pass from one medium into another. Proponents of the
theory suggest that a special force, directed
perpendicular to the interface, acts to change the speed
of the particles as they enter the second medium. The
exact nature of this force was left to speculation, and
no evidence has ever been collected to prove the theory.
Another excellent comparison of the two theories involves the differences that occur when light is reflected from a smooth, specular surface, such as a mirror. Wave theory speculates that a light source emits light waves that spread in all directions. Upon impacting a mirror, the waves are reflected according to the arrival angles, but with each wave turned back to front to produce a reversed image (Figure 4). The shape of arriving waves is strongly dependent upon how far the light source is from the mirror. Light originating from a close source still maintains a spherical, highly curved wavefront, while light emitted from a distance source will spread more and impact the mirror with wavefronts that are almost planar.
The
case for a particle nature for light is far stronger
with regards to the reflection phenomenon than it is for
refraction. Light emitted by a source, whether near or
far, arrives at the mirror surface as a stream of
particles, which bounce away or are reflected from the
smooth surface. Because the particles are very tiny, a
huge number are involved in a propagating light beam,
where they travel side by side very close together. Upon
impacting the mirror, the particles bounce from
different points, so their order in the light beam is
reversed upon reflection to produce a reversed image, as
demonstrated in Figure 4. Both the particle and wave
theories adequately explain reflection from a smooth
surface. However, the particle theory also suggests that
if the surface is very rough, the particles bounce away
at a variety of angles, scattering the light. This
theory fits very closely to experimental observation.
Particle and Wave Reflection
An excellent comparison of the wave and particle theories involves the differences that occur when light is reflected from a smooth, specular surface, such as a mirror. This interactive tutorial explores how particles and waves behave when reflected from a smooth surface.
Particles and waves should also behave differently when they encounter the edge of an object and form a shadow (Figure 5). Newton was quick to point out in his 1704 book Opticks, that "Light is never known to follow crooked passages nor to bend into the shadow". This concept is consistent with the particle theory, which proposes that light particles must always travel in straight lines. If the particles encounter the edge of a barrier, then they will cast a shadow because the particles not blocked by the barrier continue on in a straight line and cannot spread out behind the edge. On a macroscopic scale, this observation is almost correct, but it does not agree with the results obtained from light diffraction experiments on a much smaller scale.
When
light is passed through a narrow slit, the beam spreads
and becomes wider than expected. This fundamentally
important observation lends a significant amount of
credibility to the wave theory of light. Like waves in
water, light waves encountering the edge of an object
appear to bend around the edge and into its geometric
shadow, which is a region that is not directly
illuminated by the light beam. This behavior is
analogous to water waves that wrap around the end of a
raft, instead of reflecting away.
Almost a hundred years after Newton and Huygens proposed their theories, an English physicist named Thomas Young performed an experiment that strongly supported the wave-like nature of light. Because he believed that light was composed of waves, Young reasoned that some type of interaction would occur when two light waves met. In order to test this hypothesis, he used a screen containing a single, narrow slit to produce a coherent light beam (containing waves that propagate in phase) from ordinary sunlight. When the sun's rays encounter the slit, they spread out or diffract to produce a single wavefront. If this front is allowed to illuminate a second screen having two closely spaced slits, two additional sources of coherent light, perfectly in step with each other are produced (see Figure 6). Light from each slit traveling to a single point halfway between the two slits should arrive perfectly in step. The resulting waves should reinforce each other to produce a much larger wave. However, if a point on either side of the central point is considered, then light from one slit must travel much farther to reach a second point on the opposite side of the central point. Light from the slit closer to this second point would arrive before light from the distant slit, so the two waves would be out of step with each other, and might cancel each other to produce darkness.
Particle and Wave Diffraction
Particles and waves should behave differently when they encounter the edge of an object and form a shadow. This interactive tutorial explores how particles and waves behave when diffracted by an opaque surface.
As he suspected, Young discovered that when the light waves from the second set of slits are spread (or diffracted), they meet each other and overlap. In some cases, the overlap combines the two waves exactly in step. However, in other cases, the light waves are combined either slightly or completely out of step with each other. Young found that when the waves met in step, they added together by a process that has come to be termed constructive interference. Waves that meet out of step will cancel each other out, a phenomenon known as destructive interference. In between these two extremes, various degrees of constructive and destructive interference occur to produce waves having a wide spectrum of amplitudes. Young was able to observe the effects of interference on a screen placed at a set distance behind the two slits. After being diffracted, the light that is recombined by interference produces a series of bright and dark fringes along the length of the screen.
Although seemingly important, Young's conclusions were not
widely accepted at the time, primarily because of the
overwhelming belief in the particle theory. In addition to
his observations on light interference, Young postulated that
light of different colors was composed of waves having
different lengths, a fundamental concept that is widely
accepted today. In contrast, the particle theory advocates
envisioned that various colors were derived from particles
having either different masses or traveling at different
speeds.
Video
Example of Double Slit Experiment
(excerpt from What the Bleep 2)
The interference effect is not restricted to light. Waves
produced on the surface of a pool or pond will spread in all
directions and undergo an identical behavior. Where two waves
meet in step, they will add together to make a larger wave by
constructive interference. Colliding waves that are out of
step will cancel each other via destructive interference and
produce a level surface on the water.
Even more evidence for a wave-like nature of light was
uncovered when the behavior of a light beam between crossed
polarizers was carefully examined (Figure 7). Polarizing
filters have a unique molecular structure that allows only
light having a single orientation to pass through. In other
words, a polarizer can be considered a specialized type of
molecular Venetian blind having tiny rows of slats that are
oriented in a single direction within the polarizing
material. If a beam of light is allowed to impact a
polarizer, only light rays oriented parallel to the
polarizing direction are able to pass through the polarizer.
If a second polarizer is positioned behind the first and
oriented in the same direction, then light passing through
the first polarizer will also pass through the second.
The Double Slit Experiment
Explore how light waves diffracted by a twin-slit apparatus
can recombine through interference to produce a series of
dark and light fringes on a reflective screen. The tutorial
enables visitors to adjust the slit distances and alter the
resulting interference patterns.
However, if the second polarizer is rotated at a small angle,
the amount of light passing through will be decreased. When
the second polarizer is rotated so the orientation is
perpendicular to that of the first polarizer, then none of
the light passing through the first polarizer will pass
through the second. This effect is easily explained with the
wave theory, but no manipulation of the particle theory can
explain how light is blocked by the second polarizer. In
fact, the particle theory is also not adequate to explain
interference and diffraction, effects that would be later
found to be manifestations of the same phenomenon.
The effects observed with polarized light were critical to
the development of the concept that light consists of
transverse waves having components that are perpendicular to
the direction of propagation. Each of the transverse
components must have a specific orientation direction that
enables it to either pass through or to be blocked by a
polarizer. Only those waves with a transverse component
parallel to the polarizing filter will pass through, and all
others will be blocked.
By
the middle of the 1800s, scientists were becoming
increasingly convinced of the wave-like character of
light, but there remained one overbearing problem.
Exactly what is light? A breakthrough was made when it
was discovered by English physicist James Clerk Maxwell
that all forms of electromagnetic radiation represent a
continuous spectrum, and travel through a vacuum at the
same speed: 186,000 miles per second. Maxwell's
discovery effectively nailed the coffin of the particle
theory and, by the dawn of the twentieth century, it
seemed that the basic questions of light and optical
theory had finally been answered.
A major blow to the wave theory occurred behind the scenes in
the late 1880s when scientists first discovered that, under
certain conditions, light could dislodge electrons from the
atoms of several metals (Figure 8). Although at first only a
curious and unexplainable phenomenon, it was quickly
discovered that ultraviolet light could relieve atoms of
electrons in a wide variety of metals to produce a positive
electrical charge. German physicist Philipp Lenard became
interested in these observations, which he termed the
photoelectric effect. Lenard used a prism to split white
light into its component colors, and then selectively focused
each color onto a metal plate to expel electrons.
What Lenard discovered confused and amazed him. For a
specific wavelength of light (blue, for example), the
electrons produced a constant potential, or a fixed amount of
energy. Decreasing or increasing the amount of light produced
a corresponding increase or decrease in the number of
electrons liberated, but each still maintained the same
energy. In other words, electrons escaping their atomic bonds
had energies that were dependent on the wavelength of light,
not the intensity. This is contrary to what would be expected
from the wave theory. Lenard also discovered a link between
wavelength and energy: shorter wavelengths produced electrons
having greater amounts of energy.
The
foundation for a connection between light and atoms was cast
in the early 1800s when William Hyde Wollaston discovered
that the sun's spectrum was not a continuous band of light,
but contained hundreds of missing wavelengths. Over 500
narrow lines corresponding to missing wavelengths were mapped
by German physicist Joseph von Fraunhofer, who assigned
letters to the largest gaps. Later, it was discovered that
the gaps were produced from absorption of specific
wavelengths by atoms in the sun's outer layer. These
observations were some of the first links between atoms and
light, although the fundamental impact was not understood at
the time.
In
1905, Albert Einstein postulated that light might
actually have some particle characteristics, regardless
of the overwhelming evidence for a wave-like nature. In
developing his quantum theory, Einstein suggested
mathematically that electrons attached to atoms in a
metal can absorb a specific quantity of light (first
termed a quantum, but later changed to a photon) and
thus have the energy to escape. He also speculated that
if the energy of a photon were inversely proportional to
the wavelength, then shorter wavelengths would produce
electrons having higher energies, a hypothesis borne in
fact from the results of Lenard's research.
Einstein's theory was solidified in the 1920s by the
experiments of American physicist Arthur H. Compton, who
demonstrated that photons had momentum, a necessary requisite
to support the theory that matter and energy are
interchangeable. About the same time, French scientist
Louis-Victor de Broglie proposed that all matter and
radiation have properties that resemble both a particle and a
wave. De Broglie, following Max Planck's lead, extrapolated
Einstein's famous formula relating mass and energy to include
Planck's constant:
E = mc2 = hn
where E is the energy of a particle, m the mass, c is the
speed of light, h is Planck's constant, and n is the
frequency. De Broglie's work, which relates the frequency of
a wave to the energy and mass of a particle, was fundamental
in the development of a new field that would ultimately be
utilized to explain both the wave-like and particle-like
nature of light. Quantum mechanics was born from the research
of Einstein, Planck, de Broglie, Neils Bohr, Erwin
Schrödinger, and others who attempted to explain how
electromagnetic radiation can display what has now been
termed duality, or both particle-like and wave-like behavior.
At times light behaves as a particle, and at other times as a
wave. This complementary, or dual, role for the behavior of
light can be employed to describe all of the known
characteristics that have been observed experimentally,
ranging from refraction, reflection, interference, and
diffraction, to the results with polarized light and the
photoelectric effect. Combined, the properties of light work
together and allow us to observe the beauty of the universe.
Contributing
Authors
Kenneth R. Spring - Scientific Consultant, Lusby, Maryland,
20657.
Michael W. Davidson - National High Magnetic Field
Laboratory, 1800 East Paul Dirac Dr., The Florida State
University, Tallahassee, Florida, 32310.
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