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Refraction of Light - An Overview


When you look at the bottom of a bucket of water or at the clear water
in a swimming pool, the water will not appear to be as deep as it really
is. This optical illusion is due to the refraction of light. The apparent
bending of a ruler partially immersed in a container of water can also be
explained by the refraction of light.

Due to the refraction of light, a person standing waist deep in water
appears to have shortened legs, and a fisherman knows that a fish will
not be where he sees it.

If you place a 5 cent coin in the centre of the bottom of a teacup and
position yourself so that the coin is only just not visible, the coin will
come into view when water is carefully added to the teacup. When
viewed from directly overhead, the coin will always be visible on the
bottom of the teacup. Because of refraction of light, the image of the
coin and the bottom of the teacup moves when the water is added.
The refraction of light by glass is illustrated in Figure.

You will notice that some of the light is reflected. Whenever light is
refracted some is also reflected. Figure also shows that some of the
light is transmitted through the glass block. Within the glass block
you will notice that the direction of the light path has been changed.
Experiments have shown that the speed of light is different in different
media. For example, the speed of light in glass (or water) is less than
the speed of light in air. The change in the speed of light as it passes
from one medium to another explains the refraction of light.

When light is refracted by glass you will notice that the light is bent
towards the normal as the light passes from the air into the glass.
Figure shows this and also labels the incident and the refracted
rays. A normal at the point of incidence has also been drawn.
As with mirrors, i is used to denote the angle of incidence. The angle
between the refracted ray and the normal is called the angle of
refraction (r). Notice that the law of reflection is obeyed by the
reflected ray of light. The angle of deviation is the angle between the
original direction of the light and the direction after refraction.

In Figure 2.62 the ray of light passing through the semi-circular
glass block re-enters the air without any bending. This is because the
light passes through the glass along the radius of the block, which is
perpendicular to the surface of the glass. There is no bending of the
light path when the light is incident normally, i.e. is perpendicular to
the surface. In this case the radius of the block is the normal at the point
of incidence.

A similar demonstration can be set up using a semi-circular plastic
container half filled with water to show the refraction of light by water.

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