262 



HANDBOOK OF PHOTOGRAPHY 



itself, and the other is through the medium of wave motion. When our knowledge and 

 insight into the physical nature of light are more highly developed, it is reasonable to 

 believe that a single theory of light shall be developed which will embrace the experi- 

 mental evidence and theoretical considerations which are now included, separately, in 

 the wave motion and the quantum theories. 



According to the wave theory, light may be regarded as an electromagnetic 

 disturbance propagated in a hypothetical medium (called the ether) as a transverse 

 wavei with a velocity of c = 2.99796 X lO^" cm. per sec. or approximately 186,300 

 miles per sec. 



In any wave motion, the relation between the velocity of propagation c, the fre- 

 quency (or number of alternations or vibrations per unit time) v, and the wavelength 

 (or distance from a point on one wave to the corresponding point on the next adjacent 

 waves) X, is c = Xj'. Light in the visible region extends from wavelengths of from 



V/s/b/e specirum 



o o o o o o 

 o o o o oo 



LO VD 1^ CO o — 



m/i 



Wave- 

 length (X) 



Frequency(f)30 20 



10 9 8 7 6 5 



3X10 '"^Cycles 



per second 



Fig. 1. — Wavelength-frequency relationship for the visible and adjacent spectrums for 



light traveling in free space. 



400 to about 700 m/i, so that the frequency of visible light may be expected to occur 

 within the range of approximately 7.5 X 10^* to 4.3 X 10'* cycles per sec. as shown in 

 Fig. 1. The color of a given light depends upon the frequencies or wavelengths of 

 which it is composed, as well as the magnitude of the energy at these frequencies or 

 wavelengths. 



Light of only one frequency or a very small range of frequencies is said to be mono- 

 chromatic and results in a sensation of a single pure or spectral color. When more 

 than one frequency is present, the light is said to be heterochromatic, but the unaided 

 and untrained eye does not usually distinguish between monochromatic and hetero- 

 chromatic radiations in the visible spectrum. Pure monochromatic light of a single 

 frequency does not exist, although it may be very closely approached. White light 

 consists of approximately equal intensities of all frequencies within the visible spec- 

 trum. Consequently, by properly combining colored lights, it is possible to produce 

 white light. 



By permitting light to fall upon a smooth surface, the direction of propagation 

 of the light may be altered, and the light is reflected from the surface. If the reflecting 

 surface is highly polished and smooth, the reflection taking place is regular or specular 

 and is sharply defined. In this case the angle of incidence of the light to the perpen- 

 dicular of the surface always equals the angle of reflection. If the reflecting surface 

 is rough or coarse in comparison to the wavelength of light, the angles of incidence and 

 reflection for the beam as a whole may not be equal or may be only approximately 

 equal. This type of reflection is called "diffuse reflection," and the beams are not 

 sharply defined as in specular reflection. Diagrams illustrating specular and diffuse 

 reflection are shown in Fig. 2. 



I A transverse wave is one in which the displacement occurs perpendicular to the direction of 

 propagation. 



