REFRACTION
The change of direction that occurs when a ray of
light passes at an oblique angle (less than 90°) from one
transparent substance
into another substance of
different density is called refraction. Refraction occurs
because light travels at various speeds in different
transparent substances of different densities. The
greater the density of a substance, the slower the light
travels through it.
Refraction (or change of direction) always follows
a simple rule: when the light ray passes from one
transparent substance into another of greater density,
refraction is toward the normal. In this context, the
normal means a line perpendicular to the surface of the
medium at the point of entrance of the light ray. (See
fig. 5-12.) In passing from one transparent substance
into another of lesser density, refraction is away from
the normal. (See fig. 5-13.)
When a ray of light enters a denser medium at an
angle of 90°, as shown in figure 5-14, the wave fronts
slow down but remain parallel. When this same light
ray enters a denser medium at an oblique angle, the
portion of the wave front that first enters the water
moves slower than the other part of the wave front that
is still in the air. Consequently, the ray bends toward the
normal. (See fig. 5-12).
If the light ray enters a less dense medium at an
oblique angle, the ray bends away from the normal as
shown in figure 5-13. The portion of the wave front that
enters the less dense substance travels faster than the
other part of the wave front. Consequently, the ray
bends away from the normal.
When a beam of white light is passed through a
prism, as shown in figure 5-8, it is refracted and
dispersed into its component wavelengths. Each of
these wavelengths reacts differently on the eye, which
then sees the various colors that compose the visible
spectrum.
The visible spectrum ranges in color from violet at
one end to red at the other end. (See fig. 5-8.) There are
5-16
AG5f0512
NORMAL
AIR
WATER
Figure 5-12.—Wave front diagram illustrating refraction of
light at an air-water boundary. Ray is entering a more
dense substance.
AG5f0513
WATER
AIR
NORMAL
Figure 5-13.—Wave front diagram illustrating refraction of
light at an air-water boundary. Ray is entering a less dense
substance.
AG5f0514
AIR
WATER
DIRECTION OF
LIGHT RAY
Figure 5-14.—Wave front diagram illustrating the difference
in the speed of light in air and water.
