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 light rays pass through water, because we can
see through a glass of clear water; on the other hand, light rays
cannot pass through wood, leather, metal, etc.

[Illustration: FIG. 64.--A straw or stick in water seems broken.]

Whenever light meets a transparent substance obliquely, some of it is
reflected, undergoing a change in its direction; and some of it passes
onward through the medium, but the latter portion passes onward along
a new path. The ray _RO_ (Fig. 65) passes obliquely through the air to
the surface of the water, but, on entering the water, it is bent or
refracted and takes the new path _OS_. The angle _AOR_ is called the
angle of incidence. The angle _POS_ is called the angle of refraction.

[Illustration: FIG. 65.--When the ray _RO_ enters the water, its path
changes to _OS_.]

The angle of refraction is the angle formed by the refracted ray and
the perpendicular to the surface at the point where the light strikes
it.

When light passes from air into water or glass, the refracted ray is
bent toward the perpendicular, so that the angle of refraction is