LIGHT FILTERS 305 



we are looking at them at an angle rather than perpendicularly. In the right hand 

 part of Fig. 6, the lines represent the amplitudes of the three individual plane-polarized 

 waves and all are of equal length. 



A polarizing agent has the property of transmitting through it transverse waves 

 vibrating in only one plane; all other transverse vibrations are partially or completely 

 absorbed by the polarizing device. Therefore, if a polarizing device is placed in the 

 path of a beam of light which has transverse vibrations in all directions, the light 

 emerging after passing through the polarizing agent will be found to vibrate in only 

 one direction normal to the plane of propagation. The property of polarizing agents 

 to transmit light vibrating in one transverse direction but not in others is due to the 

 proper orientation of many minute crystals in these polarizing devices. 



The action of polarizing plates can be explained by analogy by considering the 

 polarizing plates to be constructed in the form of a grating of parallel bars. Wave 

 motion in such a direction as to be parallel with the bars will suffer no obstruction 

 and will readily pass through them. On the other hand wave motion perpendicular to 

 the bars will be effectively blocked. 



Many surfaces produce a greater or lesser degree of plane polarization by reflection, 

 rather than by transmission. Light, as from the sun, which is polarized in all direc- 

 tions perpendicular to its direction of travel, is more or less plane polarized after being 

 reflected from the surfaces of insulators, such as glass, wood, linoleum, etc. The 

 extent of this plane polarization which occurs in such surfaces depends upon the angle 

 of the light incident upon the surface and is a maximum for many surfaces when the 

 incident light strikes the reflecting surface at angles between 30 and 40°. The plane- 

 polarized light reflected from such surfaces is largely responsible for objectionable 

 glare. If the extent of the plane-polarized light can be reduced, the objectionable 

 glare or reflections can likewise be reduced or eliminated. 



If two polarizing plates are used one behind the other, the amount of light trans- 

 mitted through them will depend upon the relation of the optical axes of the two 

 plates. If the two polarizing agents are so aligned that their optical axes and the 

 direction of the polarizing crystals are in the same direction, maximum light will be 

 transmitted through the combination. The first crystal will, of course, cut out those 

 rays which are polarized in directions other than that corresponding to its own polar- 

 izing structure. However, since both crystals are aligned so that their crystals are 

 oriented in the same direction, the second crystal will act only in such a manner as to 

 polarize light which already comes to it plane polarized; its affect on the light passing 

 through it will therefore be simply that of absorption, as shown in Fig. 7. 



However, if the second crystal or polarizing agent has its optical axis rotated with 

 respect to that of the first crystal, the amount of light passing through the combination 

 will depend upon the angles between the optical axes of the two crystals. If this 

 angle is zero, maximum light is transmitted; if this angle is 90°, no light will be trans- 

 mitted in the case of perfect polarizing agents. For intermediate angles, intermediate 

 amounts of light will be passed. Therefore, by using two polarizing agents, one of 

 which may be rotated with respect to the other, we have a means of controlling the 

 total amount of light passing through the combination; we have also provided a means 

 of limiting the directions in which the transmitted rays are capable of vibrating. 



Use of Polarizing Plates in Photography, — In photography, polarizing crystals or 

 plates are usually used singly for the purpose of reducing glare or reflections of the 

 desired image. In most cases, elimination or reduction of reflections results in a more 

 pleasing photograph and often permits effects to be obtained which would otherwise 

 be very difficult or even impossible. The polarizing material is mounted in circular 

 cells which slip on over the lens mount. These cells are arranged so that they may 

 be rotated about the central axis of the camera in such a manner as to reduce the 



