chap, xxiv.] REFRACTION. 309 



medium into another of different density, is bent out 

 of its path at the surface of separation of the two 

 media. The deflection is called REFRACTION. Fig. 135 

 represents a ray of light passing from air into water. 

 If the ray passed perpendicularly into 

 the water, i.e. in the direction of the 

 normal NN', its course would be un- 

 affected by the water ; but when it 

 strikes the water obliquely as in- 

 dicated by the arrow, then it does not 



r Fig. 135. Refrac- 



contmue a straight course as HB, but tiou of Light. 

 is bent towards the normal as HC. Or 

 suppose c to be a bright object in the water, and CH 

 to be a ray of light reflected by it, when the ray 

 emerged from the water it would not continue a 

 straight course, but owing to the different density of the 

 air it would be bent away from the normal, and would 

 assume the direction of the line above H. An eye 

 placed at the end of that line would, therefore, receive 

 the rays proceeding from c ; since the eye always 

 refers the luminous object in the direction of the 

 rays which reach it, the eye would seem to see the 

 object c at B, displaced from its true position. 



Thus, a ray of light passing from one, medium into 

 another of greater density, is refracted towards the 

 normal ; and passing from one medium into another 

 of less density, is refracted away from the normal. 



It is refraction that causes a stick plunged 

 obliquely into water to appear bent, the immersed 

 part being raised nearer to the surface. It is re- 

 fraction also that causes the sun to appear still above 

 the horizon when it has actually sunk below it, the 

 rays from the sun being bent by the atmosphere sur- 

 rounding the earth, the sun is caused to appear 

 higher than it actually is. The mirage seen most 

 commonly in hot climates is also an effect of refrac- 

 tion. Fig. 136 shows how rays, say from a tree A, are 



