Light 185 



the concave mirror of which the front surface is hollowed out like 

 a portion of the inside surface of a hollow sphere, and the convex 

 mirror, where the front surface is curved out like a portion of 

 the outer surface of a sphere. 



The concave mirror magnifies, producing an image larger than 

 the object reflected. This type of mirror is used in telescopes, 

 in shaving glasses, and wherever a large, clear image is neces- 

 sary. Concave mirrors are used also as reflectors in automobile 

 headlights. The area of the lighted portion in the road ahead 

 can be varied by adjusting the distance of the light in front of the 

 mirror. 



The convex mirror makes a small image of a rather large field 

 in front of it. This type of mirror is often placed on automobiles 

 so that the driver may get a view of the road back of him without 

 turning to look to the rear. The image in 'such a mirror is small, 

 but it shows the entire width of the road for some distance back. 



Diffusion. Parallel rays of light are also reflected from 

 surfaces that are not smooth, such as a piece of paper. A reading 

 glass shows that even the best glazed paper is rough. In such 

 cases the rays do not bear the same relation to each other after 

 striking 1 the paper that they did before. The different rays strike 

 the uneven places in the paper at different angles and are reflected 

 in different directions. Such reflection we call diffusion of light, 

 because the rays are mixed. 



Refraction. Light waves travel in straight lines and at a 

 uniform speed as long as they continue in the same substance or 

 medium, but when they pass into some other medium the speed 

 is changed according to the density of that medium. The denser 

 the medium the more difficulty the light experiences in passing 

 through it. Light travels at the rate of 186,000 miles per second 

 in air. It travels only about three-fourths as fast in water, and 

 about two-thirds as fast in glass. On the other hand, it has been 

 discovered that light travels at a faster rate in gases that are 

 lighter than air, and still faster in a vacuum. 



When a beam of light passes at an angle from air into water, 

 it is bent downward into the liquid. This accounts for the 

 apparent bending of a stick thrust into water. Any beam of 



