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Chapter 26
Properties of Light
On the nature, speed, wavelengths, frequencies, and energies of light
The origin and nature of light have been a lively topic of discussion in physics for at
least two millennia. Plato believed that particles of light emanated from the eye. By the
late Renaissance Galileo was performing experiments to measure the speed of light. He
positioned observers on hilltops in an attempt to measure the round trip travel time of
light from lanterns. He failed and concluded that the speed of light was infinite.
The first successful measurement of the speed of light was be Ole Roemer in the late
17th century, based on observations of the orbit of Io, a moon of Jupiter. He noticed that
the timings of the orbital period were far short of average when the Earth was closest to
Jupiter and far longer than average when the Earth was far away from Jupiter. He
correctly concluded that the difference in timings was the time required by light to cross
the diameter of the Earth’s orbit.
The speed of light is close to 300,000 km/sec or 186,200 mph. Einstein postulated
that the speed of light was an absolute constant of the universe, not changing with the
motion of the source or the observer. He also proposed that the speed of light was the
absolute speed limit of the universe. Only objects with zero mass can travel that fast.
The nature of light has been argued to be particle or wave throughout history. Isaac
Newton championed the particle view and Christian Huygens the wave nature. Newton’s
views held the day until experiments by Thomas Young in 1803. As we have seen, only
waves show interference phenomena and Young’s experiments showed that light displays
interference. Light is not only a wave, but Young was able to show that light is a
transverse wave.
If light acts like a wave, it must have a medium and the name luminiferous aether was
proposed. The problem was that we knew that wave speed depended on the density of
the medium – denser media caused higher wave speed. With light moving so quickly the
density of the aether must be extreme. Yet it must permeate the universe (we see stars)
without causing any drag on objects in the motions. Clearly something was wrong with
this picture. A classic experiment in the 1880’s (the Michelson-Morley experiment)
showed that the aether does not exist. One set of problems was solved, but another took
their place – how can a wave travel through vacuum?
The culmination of the wave view of light came with the work of James Clerk
Maxwell in the late 1860’s. He showed that all electric behavior is predicted by two
equations and that all magnetic behavior by two more equations. The set of four make up
what we now call Maxwell’s Equations. He further showed that the proper combination
of an electric with a magnetic equation resulted in a wave equation. He dubbed the wave
“electromagnetic waves.” The new wave equation gave a predicted speed for E&M
(electromagnetic) waves – c, the speed of light. Later in 1885 Heinrich Hertz showed
that a newly discovered form of light (radio), generated by an electric generator, travels at
c. This solidified that connection between electromagnetic waves and light. We now
understand light to be an electric wave with a magnetic light perpendicular to it and in
phase with it.
Heinrich Hertz performed another classic experiment in 1885. In the so-called
“Photoelectric Effect” photons (light particles) are able to dislodge electrons from the
surface of a metal. Einstein gave the correct explanation in 1905 that showed that to
explain this Photoelectric Effect, light must be treated as a particle. The so-called waveparticle duality is another of the strange properties of light. We can never devise an
experiment, however, which demonstrates both wave and particle natures simultaneously.
Light comes is a very broad assortment of wavelengths. The ordering and scale of
these wavelengths is:
Region
Gamma Rays
X-rays
Ultraviolet
Visible
Infrared
Microwaves
Radio
Similar in size to
Atomic nuclei
Atoms
Viruses
Bacteria
Simple organisms
Small insects
Mountains
The order above is from shortest to longest wavelength. Since the speed of light is a
constant, the velocity equation simplifies to c = f. The frequency may be computed for
any wavelength (f = c/), frequency increasing as wavelength decreases. In 1905 Max
Planck gave the energy contained in a photon of light (E = hf, where h is Planck’s
constant). Thus energy increases as frequency increases. Gamma rays have the highest
frequency, shortest wavelength, and greatest energy.
Gamma rays and X-rays are blocked by the Earth’s atmosphere. The nitrogen and
oxygen molecules absorb these wavelengths very well. In order to observe the universe
in these wavelengths, we must lift telescopes above the atmosphere. Gamma rays have
very high energies and are quite damaging to tissue. X-rays have medical application,
since they penetrate soft tissue, but not bone.
Ultraviolet (UV) wavelengths are mostly blocked by the atmosphere, particularly at
300 nm by atmospheric ozone. The small amount of UV that comes through the
atmosphere from 300 – 400 nm (mostly from the Sun) causes cell damage which we
recognized as sun burn.
Visible light extends from 400 – 700 nm and includes the colors violet, blue, green,
yellow, orange, and red. Color is the equivalent of wavelength in this portion of the
spectrum. The cones of the retina (see below) are responsible for our color vision.
Visible photons do not have enough energy to either penetrate the soft tissue or damage
cells. Visible photons all pass through the Earth’s atmosphere.
Infrared wavelengths have still less energy and these photons of light interact with
humans by heating us up. At the longer microwave wavelengths, water is a very efficient
absorber. Microwave ovens work by using the water contained in food to absorb the
microwaves and convert their energy into heat. Infrared wavelengths will pass through
the Earth’s atmosphere or not depending on whether water vapor absorbs that particular
wavelength.
The longest wavelengths of light are called radio. The atmosphere is transparent to
virtually all radio wavelengths except the very longest. Here the ionosphere reflects the
waves. We take advantage of this reflection in AM radio transmission.
Similar to the production of sound waves, when light is produced, something is
vibrating. The smaller the wavelength of light, the smaller the system we look to for a
vibrating source. Thus infrared and microwaves are produced by molecular vibrations
and rotations, while visible, ultraviolet, and most X-rays are produced by atoms. Gamma
rays come from processes in the atomic nucleus. Only radio wavelengths defy this
simplistic view. Here the accelerations of charges produce the light.
Transparent and Opaque materials
The atoms comprising material have certain frequencies of light that they will
respond to. By analogy to the sound we just studied, we call these the natural frequencies
of the substance. Light passing through the material of these wavelengths will be easily
absorbed. The absorbed energy is converted (for the most part) into heat. The
consequence is that light near the natural frequencies is absorbed by the atoms of the
substance and is effectively blocked. The material is opaque to these wavelengths. Far
from these wavelengths, however, the light does not interact readily with the atoms of the
substance, but rather pass through. The material is transparent to these wavelengths.
Glass has many natural frequencies in the UV and is quite opaque in this region. In
visible light, however, glass is transparent.
Even in transparent material the photons of light interact with the atoms. The nature
of this interaction will become clear in chapter 30. The atomic interactions take time.
When we try to measure the speed of light in material, we see that it is slower than for
vacuum. Light travels at c between atomic interactions, but the time of the interaction
makes it appear that the light slows in the material.
Translucent materials (like frosted glass) scatter light in all directions to the extent
that information is lost. Light from an object makes it through the material, but shapes
are not readily preserved.
Shadows
The type of shadow produced by an object and a light source depend on the positions
of the source, the object, and screen. The size of the source and object also help
determine the type of shadow. An umbral shadow is the darkest part of the eclipse
shadow and results when the entire light source is blocked by the object. Point sources
will produce only umbral shadows. If the source is extended, the penumbral shadow
may appear. This lighter shadow is the result of incomplete blocking of the source by the
object.
Solar eclipses result when the moon blocks the light of the Sun and occur, therefore,
only at new moon. The umbra is small, reflecting the relatively small size of the moon.
To observe a solar eclipse, you need to travel to where the umbral shadow strikes the
Earth. If the moon is too far away for the umbral shadow to strike Earth, an annular
eclipse will be the result.
Lunar eclipses result when the shadow of the Earth blocks the light from the Moon.
Lunar eclipses occur only at full moon and are visible by anyone on the night side of the
Earth. The cooler of the eclipsed moon is a reflection of the amount of pollutants in the
Earth’s atmosphere at the time.
The Eye
The major parts of the eye that contribute to sight are:
Cornea – clear membrane covering eye that does most of the light focusing
Iris – the colored part of the eye with a central hole (pupil) that controls the amount
of light entering the eye
Lens – completes the focusing and provides focus control for wide variety of depth
Retina – light sensitive cells on the back of the eyeball. Two types exist – rods,
distributed more around the edges of the eye give perception of brightness levels;
cones, distributed more in the center of vision give color vision. Cones require a
minimum amount of light to work. Three types of cones are sensitive to different
wavelength regions and provide the color mixing required to see all colors.
Fovea – central region vision
Optic Nerve – nerve path to the brain. Blind spot exists where optic nerve connects
to the retina. Binocular vision fills in blind spot.
Myopia (nearsightedness) occurs when the light comes to focus short of the retina.
Hyperopia (farsightedness) occurs when the light has yet to reach focus at the retina.
Our brains are integral to our sight, filling in the blind spot, providing depth of field,
helping us detect motion and subtle changes in brightness.