REFRACTION 119 



medium, and that they are bent only at the point where 

 they pass from one to the other. Also we find that when 

 the rays meet the surface between the media at right 

 angles, they are not bent at all ; the angle must be 

 acute to cause refraction. Putting these facts together, 

 we may say : Refraction is the bending of light rays when 

 they pass from one medium to another of different density, 

 at an acute angle to the surface between the media. 



138. Cause of Refraction. To understand refraction, 

 it must be kept in mind that light waves travel faster in 

 a rare than in a dense 

 medium. In Fig. 91 

 let abc be the cross 

 section of a prism of 

 glass. Rays from an 

 object (the arrow) 

 move through the 



FIG. 91 



air with equal speed, 



so that when the first one reaches the glass at e' the 

 others will all have reached the line e'e. Now since 

 glass is denser than air, the first ray e 1 will move only 

 to /', while e (still in air) moves to /; thus when e 

 gets to / all the rays will have come to the line ff. 

 Through the glass they now move with equal speed, 

 but in a changed direction. The first ray to leave the 

 glass at m will now travel faster than those still within, 

 so that when the last one leaves the glass at c, the direc- 

 tion of the waves will again be changed. Note that the 

 waves are refracted on entering and on leaving the prism, 

 and both times toward its base. 



