168 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



rays. This point F is midway between the pole and C, or the 

 focal length is half the radius of curvature. 



Eefraction of Light. Up to the present the light rays have 

 been supposed to be moving through a medium of a uniform 



Fia. 78. The Principal Focus of a Spherical Mirror. 



density throughout, such as is the case when it travels through 

 air. When this is so, as we have seen, light travels in straight 

 lines, and, coming into contact with a reflecting medium, is 

 turned back, according to the laws which have been studied. 



FIG. 79. Refraction of Light. 



If, however, light passes from one medium into another of a 

 different density the propagation of the wave is no longer recti- 

 linear, the passage from one medium into the other is accom- 

 panied by a bending of its path. This bending is known as 



