PROPERTIES OF LIGHT. 



493 



along any radius meet, at intervals, with water raised most 

 above its horizontal plane as the crest of a wave, or depressed 

 most below it as the hollow of a wave. The distance along 

 the ray from crest to crest is called a wave-length and is 

 always the same in any given simple light; but differs in 

 different-colored lights; the briefer the time of an oscillation 

 the less the wave-length. 



When light falls on a polished surface separating two 

 transparent media, as air and glass, part of it is reflected 

 or turned back into the first medium; part goes on into 

 the second medium, and is commonly deviated from its 

 original course or refracted. The original ray falling on the 

 surface is the incident ray. 



Let A B (Fig. 127) be the 

 surface of separation ; a x the 

 incident ray; and CD the 

 perpendicular or normal to 

 the surface at the point of 

 incidence: a x C will then be 

 the angle of incidence. Then 

 the reflected, ray makes an 

 angle of reflection with the 

 normal which is equal to 

 the angle of incidence; and 

 the reflected ray lies in 

 the same plane as the inci- 

 dent ray and the normal to 

 the surface at x. The re- 

 fracted ray lies also in the 

 same plane as the normal and 

 the incident ray, but does not continue in its original direc- 

 tion, #/; if the medium below A B be more refractive 

 than that above it, the refracted ray is bent, as xd, nearer 

 to the normal, and making with it an angle of refraction , 

 Dxd, smaller than the angle of incidence, a.x C. If, on 

 the contrary, the second medium is less refracting than the 

 first, the refracted ray xg is bent away from the normal, 

 and makes an angle of refraction, Dxg, greater than the 

 angle of incidence. The ratio of the sine of the angle of 



FIG. 127. Diagram illustrating the 

 refraction of light. AB, surface of 

 separation between two transparent 

 media; CD, the perpendicular to the 

 surface at the point of incidence, x\ 

 ax, incident ray; xd, refracted ray, if 

 the second medium be denser than the 

 first; xg, refracted ray, if the second 

 medium is less refractive than the 

 first. The reflected ray is not repre- 

 sented, but would make an angle with 

 Gx equal to the angle axC. 



