396 



REFRACTION OF LIGHT. 



are visible are generally deeper than they seem to be. (Fig. 294.) 

 As air expands, its index of refraction becomes less. Hence the 

 indistinctness and apparent unsteadiness of objects seen through 

 air rising from the surface of a hot stove. Light is refracted as it 

 enters the earth's atmosphere. Hence the heavenly bodies appear 

 to be further above the horizon than they really are except when 

 they are overhead. 



616. Total Reflection. When a ray of light 

 passes from a rarer into a denser medium, it may always 

 approach the perpendicular so as to make the angle of re- 

 fraction less than the angle of incidence ( 614 [2]). But 

 when a ray of light attempts to pass from a denser into a 

 rarer medium there are conditions 

 under which the angle of refraction 

 cannot be greater than the angle of 

 incidence. Under such circum- 

 stances the ray cannot emerge 

 from the denser medium, but 

 mill be wholly reflected at the 

 point of incidence. Fig. 295 represents luminous rays 

 emitted from A, under water, and seeking a passage into 

 air. Passing from the perpendicular, the angle of refrac- 

 tion increases more rapidly than the angle of incidence 

 until one ray is found that emerges and grazes the surface 

 of the water. Rays beyond 

 this cannot emerge at all. 



617. The Critical An- 

 gle. Imagine a spherical 

 (Florence) flask half filled 

 with water. A ray of light 

 from L will be refracted at A 

 in the direction of R. If the 

 angle of incidence, CAL } be 



FIG. 295. 



FIG. 296. 



