216 



THE EYE 



states that for any two media, the sine of the angle of incidence 

 bears a constant ratio to the sine of the angle of refraction. 



In Fig. 46 PO is the incident. ray, OQ the refracted ray, NOM the 

 normal to the interface AB between the media (the upper being 



the less dense). Then 



sin PON 



= constant. This constant is 



sin QOM 



called the refractive index, and is usually denoted by the letter /x. 

 The refractive index of air is taken as unity, and other refractive 

 indices are expressed as numerical values compared with it. 



FIG. 47. 



If we arc given the angle of incidence and /x> we can easily 

 find the direction of the refracted ray. Let PO be the incident 

 ray. Measure OF=unity and OP=fi (stated in the same units). 

 With centre and radius OP describe a circle. Draw VS, PR 

 perpendicular to normal NOM. Draw VT parallel to ON. Join 

 TO and produce it to cut circle at Q. Then OQ is the refracted 

 ray. 



Proof : Sin PON =^, sin TOW (or QOM) = 



TO 



that is. 



Sin PON 

 Sin QOM 



PR 

 PO 



TO 

 TW 



PR 



T\\ 



PR 

 VS' 



