XI 



LIGHT 



173 



gas, a red-hot poker, or of platinum wire through which a 

 current of electricity is passing. 



It is the refraction of light in its passage from one medium 

 into another of different density which explains several very 

 familiar observations. A stick held in anything other than a 

 perpendicular position in water appears to be bent upwards 

 (Fig. 84). Standing bodies of water always appear shallower than 

 they really are on account of refraction. 



EXPT. 170. Fill a glass cylinder (Fig. 85) with water, and 

 place a coin at the bottom. On looking straight down through 

 the water the coin appears nearer the surface than it really is. 



Fig. 84. To illustrate why a Stick placed 

 slantingly in Water appears to be bent 

 upwards. 



FIG. 85. To illus- 

 trate an effect of 

 Refraction. 



Hold another coin near the outside of the cylinder and place 

 it at such a height that the two coins appear at the same level. 

 The amount by which the coin in the water is apparently 

 elevated by refraction can thus be found. The length of the 

 column of water through which the coin is observed, divided 

 by the distance from the top of the water to the outside 

 coin, gives the index of refraction of water. 

 Repeat the experiment with methylated spirit. 



Refraction through a Prism. In optics a prism is defined 

 as " a wedge-shaped piece of glass contained between two planes 



