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When a light strikes the interface between
two transparent materials, the light divides into two parts. Part of
the light ray is reflected back into the first substance, with the
angle of reflection equaling the angle of incidence. The remaining
energy in the light ray crosses the interface and enters into the
second substance.
If the incident ray strikes the glass
surface at an exact 90-degree angle, the ray goes straight into the
glass. The ray is not bent. However, if the incident ray is not at an
exact 90-degree angle to the surface, then the transmitted ray that
enters the glass is bent. The bending of the entering ray is called
refraction. How much the ray is refracted depends on the index of
refraction of the two transparent materials. If the light ray travels
from a substance whose index of refraction is smaller, into a
substance where the index of refraction is larger, the refracted ray
is bent towards the normal. If the light ray travels from a substance
where the index of refraction is larger into a substance where the
index of refraction is smaller, the refracted ray is bent away from
the normal.
Consider a light ray moving at an angle
other than 90 degrees through the boundary between glass and a
diamond.
 The glass
has an index of refraction of about 1.523. The diamond has an index of
refraction of about 2.419. Therefore, the ray that continues into the
diamond will be bent towards the normal. When that light ray crosses
the boundary between the diamond and the air at some angle other than
90 degrees, it will be bent away from the normal. The reason for this
is that air has a lower index of refraction, about 1.000 than the
index of refraction of the diamond.
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Interactive Media Activity
Interactivity: Optical Refraction
This activity shows how refraction works in different
materials (starting from a vacuum).
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