12 



HANDBOOK OF PHOTOGRAPHY 



Polarized Light. — In ordinary light, the waves are vibrating in every direction, but 

 it has been known for a long time that light reflected off the surface of glass at the 

 correct angle^ is vibrating in only one- plane. Such a light is called polarized light, 

 and has a number of interesting properties. For example, if polarized light vibrating 

 in a horizontal plane is allowed to fall upon a vertical plate of glass at the polarizing 

 angle, the whole of the light will be transmitted, and none of it will be reflected. This 

 latter case is of great use in photography as a means of reducing the specular reflection 



or "glare" from polished surfaces. 



In practice, light is polarized by means 

 of Nicol prisms made of calcite, or by 

 "polaroid" filters. Polaroid is a sheet of 

 plastic material containing certain sub- 

 microscopic crystals which transmit only 

 those vibrations taking place in one direction. 

 Light Rays. — Since the new wave front 

 formed by the Huygenian wavelets is 

 parallel to the original wave front, the 



in wavelength in passing from one medium 

 to another. 



Fig. 2. — Passage of light wa ^ ^ 



*^„°!:'?, J!fA^„^!^^„„ti':.!^°'"?? J^'jT^^ IJght win travel along lines which are every- 

 where perpendicular to the wave fronts 

 themselves. These lines representing the 

 light paths are called "rays," and almost all our discussions of lens action will be on 

 the basis of these rays, even though physically rays have no existence. Rays are 

 the analogue of a railroad track, light quanta being the trains, with the difference that 

 the quanta are in some unknown way accompanied by light waves which spread out 

 on all sides of the ray and cause interference effects with the waves belonging to the 

 neighboring rays. However, since the wavelength of the light is very small, these 

 interference effects cause, in general, only an unimportant fine structure within the 

 light distribution obtained on the assumption that each ray carries its proper share 

 of the total amount of light in the beam. We ^ 



may therefore say, in general, that where many 

 rays cross there is likely to be a strong concentra- 

 tion of light. 



The Law of Refraction. — When a train of light 

 waves falls obliquely on the surface of separation 

 between two different mediums in which the 

 speed of light is different, the parts of the waves 

 which cross the boundary will be accelerated or 

 retarded, causing the waves to take up a new 

 direction in the second medium (Fig. 3). 



Considering now the ray path, we find that it 

 is bent at the surface as shown dotted in Fig. 3, 

 the whole phenomenon being known as "refrac- 

 tion." If the angles between the ray and the normaP on left and right of the sur- 

 face are, respectively, i and i' and if the refractive indices are similarly n and n', 

 then it can be shown that these quantities are related by the equation 



n sin i = n' sin i' (2) 



which is the well-known "law of refraction." The two parts of the ray and the normal 

 all lie in one plane called the "plane of incidence." It should be noted that, because 



1 This angle of incidence is' such that its tangent is equal to the refractive index of the glass. 



2 The "normal" is a line drawn perpendicular to the refracting surface at the point where the ray 

 strikes it. 



Fig. 3. — The refraction of light. 



