Some 85% of what man learns comes to him through his vision in response
to the medium of light. Yet, ironically, it wasn’t until the end of the
17th century that he first began to get an inkling of what light really
is. It took the great scientific genius Isaac Newton to show that
so-called white light is really a combination of all the colors of the
rainbow. A few years later the Dutch astronomer Christiaan Huygens
introduced the idea that light is a wave motion, a concept finally
validated in 1803 when the British physician Thomas Young ingeniously
demonstrated interference effects in waves. Thus it was finally realized
that the only difference between the various colors of light was one of
wavelength.
For light was indeed found to be a wave phenomenon, no different in
principle from the water waves you have seen a thousand times. If you
stand at the seashore, you can easily count the number of waves that
approach the shore in a minute. Divide that number by 60 and you have
the frequency of the wave motion in the familiar unit, cycles-per-second
(cps).[1]
You would have to count pretty quickly to do this for light, however.
Light waves vibrate or oscillate at the rate of some 400 million million
times a second. That’s the vibration rate of waves of red light; violet
results from vibrations that are just about twice that fast.
With frequencies of this magnitude, discussion and handling of data and
dimensions are cumbersome and rather awkward. Fortunately there is
another approach. Let’s look again at our ocean waves. We see that there
is a regularity about them (before they begin to break on the shore).
The distance from one crest to the next is significant and is called the
_wavelength_. Water waves are measured in feet, and in comparable units
light waves are recorded in ten-millionths of an inch—again a very
cumbersome number. Scientists therefore use the metric system[2] and
have standardized a unit called the angstrom[3], which is equal to the
one-hundred-millionth part of a centimeter (10⁻⁸ cm). Thus we find, as
shown in Figure 3, that the visible light range runs from the violet at
about 4000 angstroms to red at about 7000 angstroms.
[Illustration: Figure 3 _The visible light spectrum ranges between
approximately 4000 and 7000 angstroms._]
Wavelength
(Angstroms)
Violet 4000-4300
Blue 4300-5000
Green 5000-5600
Yellow 5600-5800
Orange 5800-6100
Red 6100-7000
At roughly the same time that the wavelength of light was being
determined, the German-British astronomer William Herschel performed an
interesting experiment. He held a thermometer in turn in the various
colors of light that had been spread out by an optical prism. As he
moved the thermometer from the violet to the red, the temperature
reading rose—and it continued to rise as he moved the instrument
_beyond_ the red area, where no prismatic light could be seen.
Thus Herschel discovered infrared rays (the kind of heat we get from the
sun) adjoining the visible red light, and at the same time found that
they were merely a continuation of the visible spectrum. Shortly
thereafter, ultraviolet rays were found on the other end of the visible
light band.
One of the most fascinating movements in science has been the constant
expansion since then of both ends of the radiating-wave spectrum. The
result has come to be called the _electromagnetic spectrum_, which, as
we see in Figure 4, encompasses a wide variety of apparently different
kinds of radiation. Above the visible band (the higher frequencies), we
find ultraviolet light, X rays, gamma rays, and some cosmic rays; below
it are infrared radiation, microwaves, and radio waves. Only a small
proportion of the total spectrum is occupied by the visible band.
Another point of interest is the inverse relationship between wavelength
and frequency. As one goes up the other goes down.[4]
[Illustration: Figure 4 _Visible light region spans a tiny portion
of the total electromagnetic spectrum._]
Frequency (cps) Wavelength
Angstroms
Cosmic rays
10²² 0.0001
0.001
10²⁰ Gamma rays 0.01
0.1
10¹⁸ X rays 1
10
10¹⁶ Ultraviolet radiation 100
1,000
Visible light
10¹⁴ 10,000
Infrared radiation 100,000