164 



MINERALOGY 



an equal velocity as that from a, will strike the surface at point P 

 and will be reflected in the direction of Pb'. When the wave front 

 is at a', the ray from b will be at the point b', for the path aoa' is 

 equal to the path bPb', and the arrow will appear as at a'b', but in 

 a reverse position. If the surface of reflection is a truly plane sur- 

 face, the image of the arrow at a'b' will be of the same size and 

 shape. 



Refraction. When the ray, in Fig. 310, from a strikes the 

 surface at o, a portion of the light, depending upon the angle of 

 incidence and the character of the substance, is transmitted or pene- 

 trates the second transparent medium. The velocity and direction 

 of the entering light will be changed ; the amount of charge will 



depend upon the medium and the wave length of the entering 

 light. Suppose the upper medium to be air and the lower medium 

 to be water ; light travels approximately three quarters as fast in 

 water as in air. The ray ao, in Fig. 310, will be on the point of 

 entering the water at o, when the ray from b is at the point b" ; 

 while b is traveling the distance b"P, a will have traveled three 

 quarters of this distance in water. To find the wave front of the 

 refracted rays : draw bP parallel to ao, then with o as a center and 

 radius equal to three quarters of b"P draw the circle dT ; from P 

 draw PT tangent to the circle dT and PT will be the direction of the 

 wave front of the refracted rays. When b reaches bi, a will be at ai 

 and the arrow will appear at aibi, enlarged, but not reversed, as is 

 the case with the reflected rays. A man standing on the bank will 



