LESSONS IN PHYSICS. 



77 



incidence, BCG is the angle of refraction, and DCB is the angle of 

 deviation. The ray AC passing into the water at C is bent to- 

 ward the perpendicular FG, instead of going to D, while the ray 

 BC passing into the air at C, is bent from the perpendicular FG, 

 toward A, not going on to E. 



The cause of refraction. In. the case of ether waves, the parti- 

 cles vibrate transversely to the direction in which the waves 

 move. Now, as the waves pass obliquely from one medium into 

 another that is more dense, the end that first enters the denser 

 medium seems to move more 

 slowly, while the end that is still 

 in the rare medium moving as be- 

 fore gets a little in advance, so 

 that there is a bending of the ray 

 at the surface separating the two 

 media; much as a column of sol- 

 diers changes direction by filing 

 right or left. The reverse is true 

 as the light passes into a rarer 

 medium. (See Fig. 11.) 





FIG 1 1 



\ 



From A as a center (see Fig. 10) describe the circumference of a 

 circle and draw the lines ab and df. The line ab is the sine of the 

 angle of incidence ACF, and the line df is the sine of the angle of 

 refraction BCG. Now if we divide the line ab by the line df the 

 quotient will be what is called the index of refraction, which is 

 constant with the same substance but varies with different sub- 

 stances, lead chromate having the highest index of refraction, 

 2.97; the diamond next, 2.5, and flint glass next, 1.61. 



