Refraction of light
Refraction of light refers to the change in direction of a ray of light
when it travels from one medium to another. The change in
direction is as a result of change in speed as the light moves in the
second medium.
Light travels in a straight line as long as it is moving in the same
medium. Other waves like water waves and sound waves also
experience refraction.
The figure below shows a ray of light travelling from air to that of
glass or water
When light moves from air (rarer medium) to glass (optically
dense medium) it bends towards the normal and angle i is greater
than angle r ( i>r). Light moves faster in air (at 3 x 108 m/s) than
in glass (at 2 x108m/s )
When light moves from glass/water (optically dense medium to
air (rarer medium) to) it bends away from the normal and angle i
is less than angle r ( i<r).
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When light is incident normally ( i= 0o) it slows down but there is
no change in direction
During refraction of light:
1.
2.
3.
The frequency of the refracted ray remains constant.
Due to partial reflection and absorption of light at the
interface, the intensity of the refracted ray will be less than
the incident ray.
When the light crosses the boundary between two different
media, deviation of light occurs resulting in refraction such
that there is a change in wavelength and speed of light.
Effects of Refraction



Twinkling of stars is due to refraction of light.
Mirage and looming are optical illusions which are a result of
refraction of light.
A swimming pool always looks shallower than it really is
because the light coming from the bottom of the pool bends
at the surface due to refraction of light.
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When light is incident to a surface of a glass block, it gets
refracted as it gets into the block. The same happens as the light
crosses from the block to air again. For a glass block with parallel
sides the path of light is as below.
Refractive index
Refractive index is the measure of bending of a light ray when
passing from one medium to another.
Refractive index n is defined as the ratio of the sine of the angle of
incidence to the sine of the angle of refraction; i.e., n = sin i /
sin r.
Refractive index can also be defined as the ratio of the velocity c,
of light in empty space to velocity v in a substance, n = c/v.
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Example
1.
Light incident at an angle of 40o to a boundary between air
and glass makes an angle of refraction of 25.3o . Calculate
the refractive index of glass.
n = sin i / sin r
= sin 40 /sin 25.3
= 1.5
2.
The speed of light in air is 3x108 m/s. What is the speed of
light in a material whose refractive index is 1.5
n = velocity in air/ velocity in medium
1.5 = 3 x 108 / v
1.5v = 3 x 108
V = 3 x 108/ 1.5
V = 2 x 108 m/s.
Total Internal Reflection
This is the complete reflection of a light ray reaching an interface
with a less dense medium when the angle of incidence exceeds the
critical angle.
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When light passes from a material such as water into one of lower
refractive index such as air, there is a maximum angle of
incidence in the water that will give a refracted beam at an angle
of refraction is 90o.
critical angle C is the angle of incidence in the denser medium
corresponding to an angle of refraction of 90o in the less dense
medium.
The reason for this is clear if we consider the formulae. For an
angle of refraction of 90o we have:
2n1= sin i/ sin r
= sin c/ sin 90
= 1/1n2
For any material with refractive index is n
n = 1/sinC
Example
What is the critical angle of glass with a refractive index n = 1.5
n = 1 / sinc
1.5 = 1 /sinC
SinC =1 /1.5 = 0.67
C = 42.1o
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The conditions for total internal reflection are
i.
the light is travelling from an optically denser medium
(higher refractive index) to an optically less dense medium
(lower refractive index)
ii. the angle of incidence is greater than the critical angle.
Application of total internal reflection
The phenomenon of total internal reflection of light is used in
many optical instruments like telescopes, microscopes,
binoculars, spectroscopes, periscopes etc. The brilliance of a
diamond is due to total internal reflection.
Optical fibre works on the principle of total internal reflection
The incident light into the fibre gets repeatedly totally internally
reflected until it exits from the opposite end of glass rod. This is
used in endoscopes and telecommunications
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LENSES
Lenses can be divided into two according to their effect on ligh
 converging lenses – are fatter Iin the middle than at the
edges
 diverging lenses – are thinner in the middle than at the
edges
converging
diverging
Converging lens
It is a convex lens in which light rays that enter it parallel to its
axis converge at a single point on the opposite side.
A lens produces its focusing effect because of the refraction of a
light beam entering and emerging from the lens into the air.
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Main terms for lens:
Focal length, f is the distance between the optical centre and the
principal focus.
Optical centre is the point midway between the lens’ surfaces on
its principal axis. Rays passing through the optical centre are not
deviated.
Principal axis is the line passing symmetrically through the
optical centre of the lens.
Focal point, F is the point to which all rays close to and incident
parallel to the principal axis converge after refraction by the lens.
Focal plane is the plane which passes through the focal point
and is perpendicular to the principal axis
Formation of image by converging lens
When an illuminated object is placed on one side of the lens and a
screen (paper) placed on the other an image of the object may be
seen. This can be explained using a ray diagram
Ray diagrams
The two rays used are
i.
from top of object through the middle of the lens
undeflected
ii. from top of object parallel to the main axis of the lens. As
it passes through the lens it gets deflected to pass through
the principal focus
The point where the two cross is the position of the top of the
image. This is a real object because it forms on a screen.
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If the object is brought closer to the lens and placed between it
and F, the image forms on the same side as the object. It is said to
be virtual as it cannot form on a screen. It is upright, magnified
and further away from the lens than the object.
The rays are extended backwards to find the position of the image.
This is the form in which it is used as a magnifying glass
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