596 Appendix B 



hand medium moves more slowly than that still in the left hand medium. 

 At a still later time, it will be entirely in the second medium at D"E". 



The angle between the normal to the interface afi and the normal 

 to the incident wave Oy is called the angle of incidence 6. Likewise, 

 the angle between a/3 and the normal to the refracted wave OS is called 



Medium 

 n-c/v 



~~6 



Figure I. Refraction of a plane wave at a plane interface. 

 (This diagram is used to derive Snell's Law.) 



the angle of refraction <p. The properties of right triangles can be used 

 to show that both angles labeled d in the drawing are equal, as are both 

 angles labeled <p, and that 



FO = DO sin 6 and DD' = DO sin <p (3) 



Because FO is the distance the light ray traversed in the vacuum while 

 a similar ray was traveling DD' in the medium, one may equate the 

 times of travel. Thus 



FO 'DD' 



Using Equation 1, one may rewrite this last relationship as 



FO = nDD' 



(4) 



Combining Equations 3 and 4 and rearranging terms, one obtains 

 Snell's law 



n — sin #/sin <p (5) 



Using this law, one has only to draw the lines representing the incident 



