The subject of light as a form of radiant energy has
been theorized upon, experimented with, and studied by
many physicists and scientists. Until about three
centuries ago, no one had developed a reasonable theory
of the nature of light. Then Max Planck, a physicist,
published a theory in which light was supposed to
consist of a stream of high-speed particles. Planck
theorized that any light source sent out an untold number
of these particles. This then was the quantum theory.
The quantum
of light is called the photon. The quantum
theory is used to explain X ray, radiation, and
About the same time other physicists, Christiann
Huygens and Thomas Young, introduced a theory called
the wave motion theory. The wave motion theory is used
to explain reflection, refraction, diffraction, and
polarization. In wave motion theory, light, wavelength,
speed, and frequency are important characteristics, and
they are interrelated.
The wavelengths of light are so small that they are
measured in nanometers (nm). A nanometer is equal to
one millionth of a millimeter. Wavelengths of light range
from about 400nm to 700nm in length and travel in a
straight-line path.
The speed of light varies in different mediums. In
air, light travels about 186,000 miles per second. In a
denser medium, such as glass, light travels even more
slowly. Furthermore, in a denser medium, the speed is
different for each color of light. Wavelength is the
distance from the crest of one wave to the crest of the
1 second. The product of the
two is the speed of light.
Speed = Wavelength
frequency, or
Wavelength =
Since the speed of light in glass is slower than in air,
the wavelength must also be shorter (fig. 1-3). Only the
wavelength changes; the frequency remains constant.
Hence we identify a particular type of radiation (color
of light) by its wavelength, bearing in mind that we are
speaking of the wavelength in air.
To the photographer, there are two important
characteristics of the way light travels. First, in a given
medium, light always travels in a straight line. Second,
in a given medium, it travels at a constant speed.

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