THE SENSES 



be refracted. The first law of refraction is that the refracted ray is in 

 the same plane as the incident ray and the normal to the surface. The 

 second law is that the sine of the angle of incidence has a constant ratio 

 (for any given pair of media) to the sine of the angle of refraction. The 

 angle of incidence is the angle which the ray makes with the normal 

 to the surface, separating the two media ; the angle of refraction is the 

 angle made with the normal in the second medium. This ratio is called 

 the index of refraction between the two media. For purposes of com- 

 parison, the refractive index of a substance is usually taken as the ratio 

 of the sine of the angle of incidence to the sine of the angle of refraction 

 of a ray passing from air into the substance. 



When a ray strikes a surface at right angles, it passes through without 

 suffering refraction. When a ray parses from a less dense to a denser 

 medium (e.g., from air to water), it i.> bent towards the perpendicular. 



Fig. 389. Refraction at a Plane Surface. 

 AB is the incident; BD, the refracted 

 ray; CB, the normal to the surface. 

 When the ray passes from air into 

 another medium, the refractive index of 



the latter is the fraction . 

 sin/3 



Fig. 390. Refraction by a Medium 

 bounded by Parallel Planes, P and 

 P'. The ray ABDE issues parallel 

 to its original direction; CB, FD, 

 normals to P and P'; a, angle of 

 incidence; 8, y, angles of refraction. 



When it passes from a more dense to a less dense medium (as from 

 water to air), it is bent away from the perpendicular. 



When a ray passes across a medium bounded by parallel planes, it 

 issues parallel to itself; in other words, it undergoes no refraction 

 (Fig. 390). 



Refraction and Dispersion by a Prism. The beam of light is bent 

 towards the normal N as it passes across BA and away from the normal 

 N' as it passes across BC (Fig. 391) ; at both surfaces it is bent towards 

 the base of the prism AC. At the same time the light suffers dispersion 

 that is, the rays of shorter wave-length are more refracted than those 

 of greater wave-length. The deviation of any given ray is measured by 

 the angle which the refracted ray makes with its original direction. 

 The amount of dispersion produced by a prism is measured by the 

 difference in the deviation of the extreme rays of the spectrum. The 

 dispersion produced by a given substance is proportional to the differ- 

 ence of its refractive indices for the extreme rays. 



Refraction by a Biconvex Lens. A straight line ABC passing through 

 the centres of curvature of the two surfaces of tho lens is cn.lled the 

 principal axis. A point C lying on the principal axis between the 



