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 photoelectricity.

WAVELENGTH, SPEED, AND FREQUENCY

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 next wave (fig. 1-2). Frequency is the number of waves passing a given point in 1 second. The product of the two is the speed of light.

Therefore:

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.