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HANDBOOK OF PHOTOGRAPHY 



Before considering the use of polarizing agents as modifiers of the image rays 

 affecting the photosensitive material, it is well to obtain some idea of the nature of 

 light-wave motion, and the action of polarizing agents in general. ^ 



Light may be considered an electromagnetic wave motion, in which the vibratory 

 motion occurs at right angles to the direction of propagation. Such waves are called 

 transverse waves. Figure 1 represents a transverse wave in which the displacement 

 of the wave motion takes place only in the plane of the paper. When the wave 

 motion takes place in a single plane, as in Fig. 1, the wave motion is said to be plane 

 polarized. Ordinary light is considered to be composed of beams of plane-polarized 

 light in which the vibrations occur in all possible directions perpendicular to the direc- 

 tion of propagation. That is to say, for ordinary light, the vibrations of a transverse 

 wave occur in all possible directions in the plane perpendicular to the direction in which 

 the light beam travels. It is, of course, difficult to represent, diagrammatically, the 



Fig. 6. — Three light waves, vibrating in different directions. The left-hand diagram 

 illustrates how these waves might look as they pass the observer, while the right-hand 

 diagram illustrates how they would look when viewed "head on." Through the use of 

 polarizing plates, with optical axes aligned, only one of these vibrations, such as a-a could 

 be transmitted. 



precise state of affairs which theory and experiment indicate takes place at any one 

 particular instant of time. The best explanation of ordinary nonpolarized light is to 

 consider it to be composed of a great number of polarized waves which follow one 

 another in such rapid succession that, over any interval of time for which light effects 

 can be recorded, vibrations in all directions perpendicular to the direction of travel 

 are equally represented. Figure 6 will indicate in an elementary and simplified man- 

 ner the type of process which occurs. For simplicity a ray of ordinary light will be 

 considered to be composed of three plane-polarized light rays, c, 6, and c, the plane 

 of polarization of each of these rays being different from that of the others. For 

 simplicity it will be assumed that the amplitudes of the three plane-polarized waves 

 are all equal. Then, if we looked at the composite light wave as it passed by us and 

 were able to see the wave motion of the separate individual plane-polarized waves, the 

 effect would be somewhat as indicated by the left diagram. On the other hand, if we 

 could see the vibrations of the three plane-polarized beams as they come directly 

 to us (or, e.g., as evaluated by a photographic plate) separately and individually, 

 the effect of the three plane-polarized waves would be somewhat as indicated at the 

 right which is an end projection. Actually, of course, ordinary light is composed of 

 many more plane-polarized waves and the net picture is much more complicated 

 than is indicated here, although the same fundamental principles apply. 



In Fig. 6, the horizontal line is the zero axis and the waves are assumed to be 

 traveling from left to right. The distance from this zero axis represents the wave 

 displacement. The waves h and c appear to have less amplitude than wave a because 



1 A fairly extensive treatment of this subject may be found in Chap. 29 of " The Principles of Optics," 

 by A. C. Hardy and F. H. Perrin, McGraw. 



