There are two sources of lightnatural and artificial. Nearly all natural light is received from the Sun. Artificial light is light such as that pro-duced by electric lamps, fires, or fluorescent tubes. 

Luminous bodies are those bodies which pro-duce their own light, such as the Sun and the stars. Illuminated or nonluminous bodies are those bodies which merely reflect the light they receive and are therefore visible because of this reflec-tion. The Moon is an example of an illuminated body.

When light is emitted from a source, waves of radiation travel in straight lines and in all directions. A simple example of motion similar to that of radiation waves can be seen by drop-ping a pebble into a pool of water. The waves spread out in expanding circles; similarly, light waves spread out in all directions to form a sphere. The boundary formed by each wave is called a wave front. Lines, or rays, drawn from the light source to any point on one of these waves indicate the direction in which the wave fronts are moving. Light radiates from its source in all direc-tions until absorbed or diverted by coming in con-tact with some substance or object.

The wavelength of a light wave is the distance from the crest of one wave to the crest of the following wave. Wavelength, frequency (the number of waves which pass a given point in a unit of time), and speed are related by the simple equation:

Where:

Because the speed of electromagnetic energy is constant, the frequency must increase if the wavelength decreases and vice versa. Wavelength is measured in ANGSTROM UNITS (A). They may also be measured in millimicrons, or millionths of millimeters (ml). Figures 5-3-1 and 5-3-2 show the visible and in-visible spectrums colors in relation to their wavelengths. Figure 5-3-2 shows that the visible spectrum occupies only a small portion of the complete electromagnetic spectrum extending between 4,000 and 7,000 angstroms only.

When light waves encounter any substance, they are either reflected, absorbed, or refracted. (See fig. 5-3-3.)

Substances that permit the penetration of clear vision through them and transmit almost all the light falling upon them, such as glass and air, are transparent. There is no known substance that is perfectly transparent, but many are nearly so. Those substances that allow the passage of part of the light but appear clouded and impair vision substantially, such as frosted light bulbs, are con-sidered translucent. Those substances that do not transmit any light are termed opaque. All objects that are not light sources are visi-ble only because they reflect all or some part of the light reaching them from a luminous source. If light is neither refracted nor reflected, it is absorbed or taken up by the medium. When light strikes a substance, some absorption and some reflection always takes place. No substance com-pletely refracts (transmits), reflects, or absorbs all the light that reaches its surface. Figure 5-3-3 illustrates this refraction, absorption, and reflec-tion of light using a flat pane of glass.

Figure 5-3-1.Wavelength of various visible and invisible colors.

Figure 5-3-2.Wavelengths and refraction.

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