this same perpendicular is called the angle of reflection.  Observation and experiment have taught us that light is always reflected in such a way that the angle of reflection equals the angle of incidence.  Light is not the only illustration we have of the law of reflection.  Every child who bounces a ball makes use of this law, but he uses it unconsciously.  If an elastic ball is thrown perpendicularly against the floor, it returns to the sender; if it is thrown against the floor at an angle (Fig. 61), it rebounds in the opposite direction, but always in such a way that the angle of reflection equals the angle of incidence.

[Illustration:  FIG. 60.—­The ray AC is reflected as CD.]

[Illustration:  FIG. 61.—­A bouncing ball illustrates the law of reflection.]

105.  Why the Image seems to be behind the Mirror.  If a candle is placed in front of a mirror, as in Figure 62, one of the rays of light which leaves the candle will fall upon the mirror as AB and will be reflected as BC (in such a way that the angle of reflection equals the angle of incidence).  If an observer stands at C, he will think that the point A of the candle is somewhere along the line CB extended.  Such a supposition would be justified from Section 102.  But the candle sends out light in all directions; one ray therefore will strike the mirror as AD and will be reflected as DE, and an observer at E will think that the point A of the candle is somewhere along the line ED.  In order that both observers may be correct, that is, in order that the light may seem to be in both these directions, the image of the point A must seem to be at the intersection of the two lines.  In a similar manner it can be shown that every point of the image of the candle seems to be behind the mirror.

[Illustration:  FIG. 62.—­The image is a duplicate of the object, but appears to be behind the mirror.]

It can be shown by experiment that the distance of the image behind the mirror is equal to the distance of the object in front of the mirror.

106.  Why Objects are Visible.  If the beam of light falls upon a sheet of paper, or upon a photograph, instead of upon a smooth polished surface, no definite reflected ray will be seen, but a glare will be produced by the scattering of the beam of light.  The surface of the paper or photograph is rough, and as a result, it scatters the beam in every direction.  It is hard for us to realize that a smooth sheet of paper is by no means so smooth as it looks.  It is rough compared with a polished mirror.  The law of reflection always holds, however, no matter what the reflecting surface is,—­the angle of reflection always equals the angle of incidence.  In a smooth body the reflected beams are all parallel; in a rough body, the reflected beams are inclined to each other in all sorts of ways, and no two beams leave the paper in exactly the same direction.