518 



THE HUMAN BODY. 



lights of different colors; 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. 145) be the 

 surface of separation; a x the 

 incident ray; and C D the 

 perpendicular or normal to the 

 surface at the point of inci- 

 dence : 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 incident ray and the nor- 

 mal to the surface at x. The 

 refracted ray lies also in the 

 same plane as the normal and 

 equal to the angle a x c. fo Q indent ray, but does not 



continue in its original direction, x /; if the medium below 

 A B be more refractive than that above it, the refracted ray 

 is bent, as x d, nearer to the normal, and making with it an 

 angle of refraction, D xd, smaller than the angle of inci- 

 dence, a x C. If, on the contrary, the second medium is less 

 refracting than the first, the refracted ray x g is bent away 

 from the normal, and makes an angle of refraction, D x g, 

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

 the angle of incidence to that of the angle of refraction is 

 always the same for the same two media with light of the 

 same wave-length. When the first medium is air the ratio of 

 the sine of the angle of refraction to that of the angle of in- 

 cidence is called the refractive index of the second medium. 

 The greater this refractive index the more is the refracted 

 ray deviated from its original course. Rays which fall per- 

 pendicularly on the surface of separation of two media pass 

 on without refraction. 



D 



FIG. M 45. Diagram illustrating the 

 refraction of light. A B, surface of 

 separation between two transparent 

 media; C D, the perpendicular to the 

 surface at the point of incidence, #; 

 a x, incident ray; x d, refracted ray, 

 if the second medium be denser than 

 the first ; x <?, refracted ray, if the 

 second medium is less refractive than 

 the first. The reflected ray is not 



with 



