path by which it came. If, however, the light makes an angle with the
mirror, its direction is changed, and it leaves the mirror along a new
path. By observation we learn that when a beam strikes the mirror and
makes an angle of 30° with the perpendicular, the beam is reflected in
such a way that its new path also makes an angle of 30° with the
perpendicular. If the sunbeam strikes the mirror at an angle of 32°
with the perpendicular, the path of the reflected ray also makes an
angle of 32° with the perpendicular. The ray (_AC_, Fig. 60) which
falls upon the mirror is called the incident ray, and the angle which
the incident ray (_AC_) makes with the perpendicular (_BC_) to the
mirror, at the point where the ray strikes the mirror, is called the
angle of incidence. The angle formed by the reflected ray (_CD_) and
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