272 



REFLECTION AND REFRACTION OF LIGHT [CH. IX 



If the surface is irregular the reflection will also be irregular and 

 the light will be reflected from the point of incidence in the form of a 



hemisphere (fig. 153), hence 

 light would reach the eye from 

 any point in the hemisphere. 

 446. Refraction. As or- 

 dinarily considered, this is the 

 change in direction which light 

 undergoes when passing ob- 

 liquely from one transparent 

 medium into another (fig. 1 54- 



FIG. 152. REGULAR OR MIRROR 

 REFLECTION. 



(From Optic Projection). 



The angle of incidence i, is equal to the 

 angle of reflection r\ and the incident and 

 reflected ray are in a plane perpendicular to 

 the reflecting surface. 



A broader statement cover- 

 ing all the phenomena whether 

 the ray passes obliquely or 

 normally from one medium to 

 another is this: Refraction is the change in velocity of the waves of 

 light in passing from one transparent medium into another. 



446. Law of refraction. The amount of bending depends upon 

 two factors, the relative density of the two media and the obliquity 

 of the incident light. The greater the ob- 

 liquity of the incident ray, and the greater 

 the difference in density, the greater will 

 be the refraction. The precise law govern- 

 ing the course and relation of the ray in 

 the two media is known as the sine law of 

 Snell and Descartes. It is expressed thus: 



sin. i 

 sin r 



= index of refraction. That is, the 



sine of the angle of the incident ray with 



the normal, divided by the sine of the angle 



of the refracted ray with its normal, gives 



the relative direction of the ray in the two 



media, i.e., the index of refraction. For 



example in fig. 154, showing the passage of light to water, the ray 



being at 60 with the normal in air, and 40 38' in water, the real 



FIG. 153. IRREGULAR OR 

 DIFFUSE REFLECTION'. 



(From Optic Projection). 



A ray of light meeting a 

 rough surface, like a piece of 

 white paper, is scattered al- 

 most equally in all direc- 

 tions, making a hemisphere 

 of light. 



