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The Baseline
Light: Particle or Wave?
The issue of whether light is a particle or a wave was an important historical debate.
The actual answer is still unnerving to some people. Here, we will explore the history of
this question and present the modern interpretation of the nature of light.
David W. Ball
I
s light a particle or a wave? Not only was this an important question for the modern understanding of the
world around us, but the answer was long in coming.
Particles and Waves
It should be obvious that matter — anything that has mass
and takes up space — is ultimately particulate. In fact, matter defines what it is to be a particle. Particles come in
pieces that can be broken into smaller pieces, and smaller
pieces can be fused together to make larger pieces. No two
pieces can occupy the same exact point in space, a concept
that applies at the atomic and molecular level for matter
that is a solution, a liquid, or a gas. Matter in motion tends
to move in a straight line unless some force acts on it, and
matter in motion tends to follow Newton’s laws of motion
(which can be reformulated into Lagrange’s or Hamilton’s
laws of motion). Moving matter can be blocked by the presence of other matter, much like an umbrella blocks the
paths of raindrops falling on it. Moving matter also can
bounce off of surfaces, as demonstrated by many a basketball player. See Figure 1.
Waves behave differently. Waves (mechanical waves, at
least) are a regular disturbance of some medium through
which the wave travels and transfers energy. The particles of
the medium oscillate regularly either in the direction of the
wave (longitudinal waves) or perpendicular to the direction
of the wave (transverse waves). A particular wave can be described by its wavelength, the distance between two similar
amplitudes of consecutive waves, and its frequency, the
number of waves passing a fixed point in space per second.
The product of the wave’s frequency and wavelength is the
wave’s velocity. Waves can combine, a process called inter-
ference. If similar amplitudes of two waves combine to
make a larger wave, the waves are constructively interfering.
If opposite amplitudes of two waves combine to cancel each
other out, the waves are destructively interfering. Waves can
diffract: they can change their direction in their medium,
which is why you can hear someone yelling around a corner. Waves can be reflected from surfaces (think of an echo
you might hear off of a far building). See Figure 2.
What is Light?
Is light a particle or a wave? The actual nature of light is a
topic that has occupied minds for millennia. The earliest
mention of the nature of light might have been by
Pythagorus (of Pythagorean theorem fame; ca. 580–500
B.C.), whose particle hypothesis of light suggested that light
particles were emitted by all objects and were intercepted by
the eyes (called the “intromission hypothesis” of light).
However, Plato (ca. 427–347 B.C.) suggested that light was
created by the eye (the “emission hypothesis” of light) and
illuminated other objects. In contrast, Plato’s pupil Aristotle
(384–322 B.C.) thought that light was some kind of wave.
In about 300 B.C., the mathematician Euclid noted that
light traveled in straight lines and obeyed a law of reflection, the concept that the angle of reflection equals the
angle of incidence. Euclid also thought that light was a wave
that was emitted by the eye, and light waves must move extremely fast because when you open your eyes, objects —
even faraway objects — are immediately visible. (The determination of the speed of light is another fascinating story,
but that’s another column.) In the 10th century A.D., Persian scientist Alhazen Abu Ali al-Hasan Ibn Al-Haitham
used experiments to support the intromission hypothesis of
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21(6) Spectroscopy 31
Figure 1: One property of matter is that no two pieces of matter can fit in the same point in
space. This momentum-based toy, called Newton’s cradle, is based on that property.
Figure 2: Waves have certain properties that matter does not: wavelength, frequency,
interference, and other properties. Permission to reprint courtesy Dr. Andrew Davidhazy, School
of Photographic Arts and Sciences, Rochester Institute of Technology.
light, but apparently did not address
the “particle versus wave” nature of
light.
The issue of particle versus wave
came to a crescendo during the European Renaissance. In 1621, Dutch
physicist Willebrod Snell announced
Snell’s law of refraction, suggesting
that light is a wave. A wave nature of
light also was espoused by English
physicist Robert Hooke, and in 1690,
Dutch scientist Christiaan Huygens
(Figure 3) published Traite de Lumiere
(“Treatise on Light”), which not only
supported the concept of light as a
wave, but used the wave nature of light
to explain many of its properties, including refraction and polarization (a
property of light originally discovered
by Danish scientist Rasmus Bartholin
in 1669). It was the most comprehensive wave description of light to date.
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Figure 3: Dutch scientist Christiaan Huygens
(1629–1695) was an early proponent of light
existing as a wave.
Figure 4: English polymath Isaac Newton
(1643–1727) argued that light was a particle.
Given his scientific stature, his viewpoint won
many converts.
However, that is not what Young observed. (Young actually used a thin
card to split a narrow beam of light to
simulate two closely spaced slits.)
Young saw an intense image at a point
between the two slits, then dark fringes
on either side, then more bright images, then dark images, and so forth
(Figure 6). In short, Young was observing constructive and destructive interference. Matter certainly does not constructively and destructively interfere
(right?), so light must be a wave. Similar work by Augustin Fresnel and, ultimately, James Clerk Maxwell’s 1864
summary of the four equations of
electromagnetism (Maxwell’s equations) cemented light’s nature as a
wave.
What Light Is — Part II
What Light Is — Part I
In 1801, English scientist Thomas
Young performed what became known
as the double-slit experiment. If a beam
of light were passed through a narrow
slit and projected onto a screen, a single image would appear, getting less
intense as one moved laterally away
from the point directly opposite the
slit (Figure 5). What would happen if
there were two closely spaced slits? If
light were a particle, one would see
two closely spaced images on the
screen.
Incoming light
Slit
Screen
Intensity of projected light
Isaac Newton (Figure 4), however,
was impressed by the fact that light
casts sharp shadows, as if it were a
stream of particles being blocked. He
favored a corpuscular nature of light,
which he also used to explain reflection and, albeit with difficulty, refraction. Such was Newton’s fame that by
virtue of his influence, the true nature
of light was still uncertain until the
early 1800s, although many people
were convinced that Newton was
correct.
Figure 5: If light is passed through a single slit, a single image with the illustrated intensity
profile will appear on a screen. This is what would be expected if light were either a particle or
a wave.
But wait, there’s more. In the late
1800s, researchers noted certain behaviors of light (blackbody light, in
particular) that did not conform to the
currently accepted understanding of
nature. Such behaviors included the
intensity distribution of emitted light
versus wavelength and temperature;
variation of emitted wavelength maxima with temperature; and total power
emitted. Attempts to explain the behavior of light theoretically ultimately
failed. Then, in 1900, German thermodynamicist Max Planck proposed that
light emitted from blackbodies originated from oscillators in matter whose
energies E were proportional to the
frequencies of their oscillations; the
modern symbol for the proportionality constant is h, and is called Planck’s
constant:
E h
where , the Greek letter nu, is used
to represent frequency. Because these
matter oscillators are limited to having
a certain quantity of energy, they are
said to be quantized. After making this
assumption, Planck was able to use
statistical thermodynamical arguments
to derive an equation that fit the intensity distribution of blackbody radiation. Although there was now a theoretical basis for the behavior of light,
even Planck was on record as being
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light also has a certain amount of energy depending upon its frequency,
and it also has momentum. In those
respects, it acts like a particle.
Some speak of a “particle-wave duality”; others even use a term I shudder
at: “wavicle.” Perhaps the best perspective is to understand that being a particle or a wave is no longer mutually exclusive. This was reinforced in the
mid-1920s, when the modern theory
of electron behavior — quantum mechanics — was based upon these tiny
pieces of matter acting as waves.
Intensity of projected light
Incoming light
Slit
Screen
Figure 6: Light passed through two closely spaced slits shows the illustrated intensity pattern on
a screen. This pattern indicates constructive and destructive interference, and (supposedly)
demonstrates that light is a wave.
uncertain about quantized energy of
his oscillators.
However, in 1905, German physicist
Albert Einstein, a Swiss patent examiner at the time, used Planck’s “E h” equation and applied it to light itself, suggesting that light had an
amount of energy proportional to its
frequency. With that interpretation of
light, Einstein was able to explain the
photoelectric effect, an effect in which
electrons were emitted from metals
when light was shined on them. The
photoelectric effect was another phenomenon that previous science was
unable to explain, but Einstein showed
that if the energy of light were indeed
quantized, the photoelectric effect had
a straightforward explanation. Thus,
Einstein was able to rationalize
Planck’s proposal using a completely
different phenomenon.
(Science now recognizes that
Planck’s original assertion was so
groundbreaking that science is broken
historically into two time periods: pre1900, or Classical Physics, and post1900, Modern Physics.)
Einstein’s interpretation of light,
then, is that it acts like a particle of energy; these particles were eventually
named photons. Later, in 1922, American physicist Arthur Compton demonstrated that photons also have momentum, another particle property. It
has now been firmly established that
light has particle properties, re-awak-
21(6) Spectroscopy 33
ening the earlier arguments of light as
a particle.
So is light a particle or a wave? It has
properties of both. Light can be labeled with a wavelength, frequency, velocity; it can reflect, refract, interfere,
and diffract. In those (and other) respects, light behaves like a wave. But
David W. Ball is
a professor of chemistry at Cleveland State
University in Ohio.
Many of his “Baseline”
columns have been
reprinted in book form
by SPIE Press as The
Basics of Spectroscopy,
available through the
SPIE Web Bookstore at
www.spie.org. His most recent book, Field
Guide to Spectroscopy (published in May
2006), is available from SPIE Press. He can
be reached at [email protected]; his
webiste is academic.csuohio.edu/ball.
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