394 REFRACTION OF LIGHT. 



shall fall upon the surface of the water at t, the intersection of the 

 two ink-marks. Notice that the reflected beam is straight until it 

 reaches the water, but that it is bent as it obliquely enters the 

 water. 



612. Refraction. Refraction of light is the 

 bending of a luminous ray when it passes from 

 one medium to another. 



613. Index of Refraction. If a ray of light from 

 L (Fig. 293) fall upon the surface of water at A, it will be 

 refracted as shown in the figure. The angle LAB is the 

 angle of incidence and KAC the angle of refraction, BC 

 being perpendicular to the water's surface. From A as a 



centre, with a radius equal to unity, 

 describe a circle. From the points m 

 and p, where this circle cuts the inci- 

 dent and refracted rays, draw mn and 

 pq perpendicular to BC. Then will 

 mn be the sine of the angle of incidence 

 and pq the sine of the angle of refrac- 

 tion. The quotient arising from 

 dividing the sine of the angle of 

 incidence by the sine of the angle of refraction is 

 called the index of refraction for the two media. 

 It is evident that the greater the refractive power of the 

 substance, the less the value of the divisor pq, and the 

 greater the value of the quotient, the index of refrac- 

 tion. 



(a.) The following table gives the indices of refraction when light 

 passes from a vacuum into any of the substances named : 



Air 1.000294 



Water 1.336 



Alcohol 1.374 



Crown glass 1.534 



Flint glass 1.575 



Carbon bisulphide 1.678 



Diamond 2.439 



Lead chromate 2.974 



