156 



LIGHT AND SOUND 



FIG. 146. 



The angle of incidence is equal to the angle of 

 reflection. 



Usually two or more of these results take place at the 

 same time. 



174. Reflection of Light. If you throw an elastic 

 ball, such as a tennis or golf ball, vertically downward 

 against a horizontal sidewalk, you expect it to bound back 

 vertically upward. But if you throw the ball downward in 

 an inclined direction, as A C of Fig. 146, it bounds off in 



another inclined direc- 

 tion CB. Except for the 

 effect of gravity on the 

 ball (cf. 21) the angle 

 \/ made by AC with CD 



(angle A CD) is of the 

 same size as the angle 

 made by CB with CD 

 (angle BCD). The first angle (A CD) is called the angle 

 of incidence (i. e., falling upon); BCD is the angle of 

 reflection (i.e., bending back). The rule is: 

 The angle of incidence equals the angle of reflection. 

 Light and sound are reflected in the same way as the 

 ball. If A of Fig. 146 represents a small opening through 

 which a beam of light is admitted into a dark room, and 

 if the reflecting surface at C is a smooth plane mirror, you 

 will see the reflected beam only when your eye is placed 

 somewhere on the line CB. 



175. Mirrors. A mirror, or looking-glass, is a good 

 reflector of light, because at its back there is a thin layer 

 of a metal, usually silver. You have noticed that when you 

 look at any object, including yourself, in a mirror, the right- 

 hand side of the object becomes the left-hand side of the 



