ight 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
smaller than the angle of incidence. When a ray of light passes from
water or glass into air, the refracted ray is bent away from the
perpendicular so that the angle of refraction is greater than the
angle of incidence.

The bending or deviation of light in its passage from one substance to
another is called refraction.

108. How Refraction Deceives us. Refraction is the source of many
illusions; bent rays of light make objects appear where they really
are not. A fish at _A_ (Fig. 66) seems to be at _B_. The end of the
stick in Figure 64 seems to be nearer the surface of the water than it
really is.

[Illustration: FIG. 66.--A fish at _A_ seems to be at _B_.]

The light from the sun, moon, and stars can reach us only by passing
through the atmosphere, but in Section 76, we learned that the
atmosphere varies in density from level to level; hence all the light
which travels through the atmosphere is constantly deviated from its
original path, and before the light reaches the eye it has undergone
many changes in direction. Now we learned in Section 102, that the
direction of the rays of light as they enter the eye determines the
direction in which an object is seen; hence the sun, moon, and stars
seem to be along the lines which enter the eye, although in reality
they are not.

109. Uses of Refraction. If it were not for refraction, or the
devia