Theory When  light  is  emitted  from  a  source,  waves  of radiation travel in straight lines and in all directions. Dropping a pebble into a pool of water can see a simple example of motion, similar to that of radiation waves. 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 directions until absorbed or diverted by coming in contact with some substance or object. Wavelength 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: C = lF Where: C = speed l = wavelength F = frequency 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  (mA).  Figures  5-7  and  5-8 show  the  visible  and  invisible  spectrum’s  colors  in relation to their wavelengths. Figure 5-8 shows that the visible  spectrum  occupies  only  a  small  portion  of  the complete electromagnetic spectrum extending between 4,000 and 7,000 angstroms only. 5-14 L L L L L L L L ULTRAVIOLET VIOLET BLUE GREEN YELLOW ORANGE RED INFRARED AG5f0507 Figure 5-7.—Wavelength of various visible and invisible colors. AG5f0508 REFRACTION OF LIGHT BY A PRISM. THE LONGEST RAYS ARE INFRARED;   THE SHORTEST, ULTRAVIOLET. 10 10 10 10 10 10 10 10 10 10 10 -6 -4 -2 2 4 6 8 10 12 14 16 1 COSMIC RAYS GAMMA RAYS X RAYS ULTRA- VIOLET RAYS INFRA- RED RAYS HERTZIAN WAVES RADIO WAVES LONG ELECTRICAL OSCILLATIONS VISIBLE SPECTRUM WAVELENGTHS IN MILLIMICRONS Figure 5-8.—Wavelengths and refraction.