AEROGRAPHER'S MATE 3 & 2 



to these radiation waves can be made by 

 dropping a pebble into a pool of water. The 

 waves spread out in expanding circles. 

 Actually, light waves spreading in all directions 

 form a sphere instead of a simple circle. The 

 boundary formed by each wave is called a 

 "wave front." 



Lines drawn from the light source to any 

 point on one of these waves indicate the 

 direction in which the wave fronts are moving. 

 These lines are the radii of the spheres formed 

 by the wave fronts and are called light rays. 



Light radiates from its source in all 

 directions until absorbed or diverted by coming 

 in contact with some substance. 



Wavelength 



Wavelength means the distance from the 

 crest of one wave to the crest of the following 

 wave, or the distance from one corresponding 

 point on one wave to a corresponding point 

 on the next 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 = XF 



Where: 



C = speed 



X = wavelength 



F = frequency 



Since the speed of electromagnetic energy is 

 constant, the frequency must increase if the 

 wavelength decreases and vice versa. 



Wavelengths are measured in ANGSTROM 

 UNITS, or more usually, an Angstrom (A). 

 They may also be measured in millimicrons 

 which are millionths of millimeters; the symbol 

 is mM . In figures 12-13 and 12-14 we can see 

 the visible and invisible spectrum's colors in 

 relation to their wavelengths. Figure 12-14 shows 

 the visible spectrum of wavelengths only occupies 

 a very small portion of the complete electro- 

 magnetic spectrum and that it only extends 

 between 4,000 and 7,000 angstroms. 



Characteristics 



When light waves encounter any substance, 

 they are either transmitted, reflected, or 

 absorbed. (See fig. 12-15.) 



•4- 



209.13 

 Figure 12-13, — Wavelength of various visible 

 and invisible colors. 



Those substances which permit the penetration 

 of clear vision through them, and which transmit 

 almost all the light falling upon them are said 

 to be transparent; air and glass are examples. 

 There is no substance known which is perfectly 

 transparent, but many substances are nearly so. 

 Those substances which allow the passage of 

 part of the light but appear clouded and impair 

 vision substantially, are called translucent; 

 frosted electric lamps and parchment shades 

 are examples. Those substances which do not 

 transmit any light are called opaque. 



All objects which are not light sources are 

 visible only because they reflect all or some 

 part of the light reaching them from some 

 luminous source. If light is neither transmitted 

 nor reflected, it is absorbed or taken up by 

 the medium. When light strikes a substance, 

 some absorption and reflection always take place. 

 No substance completely transmits, reflects, 

 or absorbs all the light which reaches its 

 surface. Figure 12-15 shows how glass trans- 

 mits, absorbs, and reflects light. 



Candlepower and Foot-Candles 



Illumination is the light received from a 

 light source. The intensity of illumination is 

 measured in foot-candles. A foot-candle is the 

 amount of light falling upon a surface 1 square 

 foot in area, 1 foot away from the light source 

 of 1 candle-power. 



280 



