398 



REFRACTION OF LIGHT. 



placed at m and n. In this way a ray may be reflected at m, n and o, 

 and refracted at i. 



(e.) Fig. 298 represents a glass jar with an opening, from which 



a stream of water issues under a 

 head ( 254 [a]) kept constant. 

 Through a lens placed opposite 

 this orifice, a concentrated beam 

 of light from the heliostat is 

 thrown into the stream of water 

 as it issues. Internal reflection 

 keeps most of it there, a prisoner. 

 The stream of water is full of 

 light and appears a stream of 

 melted metal. Thrust a finger 

 into the stream and notice the 

 effect. Place a piece of red glass 

 between the heliostat and the 

 lens ; the water looks like blood. 

 Thrust the finger into the stream again. Repeat the experiment 

 with pieces of glass of other colors in place of the red. 



FIG. 298. 



618. Refraction Explained. To understand the 

 way in which a ray of light is refracted, let us consider its 

 passage through a glass prism, ABC. It must be under- 

 stood that the velocity of light is 

 less in glass than in air, and 

 that the direction in which a 

 wave moves is perpendicular to 

 its wave front. A wave in the 

 ether approaches the surface of the 

 prism AB. When at a, the lower end of the wave front 

 first strikes the glass and enters it. The progress of this 

 end of the wave front, being slower than that of the other 

 which is still in the air, is continually retarded until the 

 whole front has entered the glass. The wave front thus 

 assumes the position shown at c. But the path of the 

 wave being perpendicular to the front of the wave, this 



FIG. 299. 



