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.

[Illustration: FIG. 63.--The surface of the paper, although smooth in
appearance, is in reality rough, and scatters the light in every
direction.]

Hot coals, red-hot stoves, gas flames, and candles shine by their own
light, and are self-luminous. Objects like chairs, tables, carpets,
have no light within themselves and are visible only when they receive
light from a luminous source and reflect that light. We know that
these objects are not self-luminous, because they are not visible at
night unless a lamp or gas is burning. When light from any luminous
object falls upon books, desks, or dishes, it meets rough surfaces,
and hence undergoes diffuse reflection, and is scattered irregularly
in all directions. No matter where the eye is, some reflected rays
enter it, and the various objects are clearly seen.

CHAPTER XI

REFRACTION

107. Bent Rays of Light. A straw in a glass of lemonade seems to be
broken at the surface of the liquid, the handle of a teaspoon in a cup
of water appears broken, and objects seen through a glass of water may
seem distorted and changed in size. When light passes from air into
water, or from any transparent substance into another of different
density, its direction is changed, and it emerges along an entirely
new path (Fig. 64). We know that light rays pass through glass,
because we can see through the window panes and through our
spectacles; we know that l