ith the perpendicular, the beam is reflected in
such a way that its new path also makes an angle of 30 deg. with the
perpendicular. If the sunbeam strikes the mirror at an angle of 32 deg.
with the perpendicular, the path of the reflected ray also makes an
angle of 32 deg. 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
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,