REFRACTION ON PLANE SURFACES
REFRACTION
ON PLANE SURFACES
Concepts of Refraction.
Refraction
is the bending of a wave when it enters a medium where its speed is different.
We can define it as the change in direction of a wave passing from one medium
to another or from gradual change in the medium.
Causes of Refraction
i.
The frequency of the refracted ray
remains constant.
ii.
Due to the partial reflection and
absorption of light at the interface, the intensity of the refracted ray will
be less than the incident ray.
iii.
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.
Examples
are: I. A swimming pool seems much
shallower than it actually is
II. a spoon appears bent when part of it is in water
III. a boy's legs look shorter when immersed
in pool. All these effects are due to the refraction of light.
Laws of Refraction.
There
are two laws governing the refraction of light:
I.
The incident ray, the refracted ray
and the normal at the point of incidence all lie on the same plane.
II.
The ratio of the sine of the angle of
incidence to the sine of angle of refraction is a constant for a given pair of
media. The second law is known as Snell’s law. n = sin i / sin r (a
constant) for a given pair of media.
If light is travelling from air to glass ang = sine of angle of incidence on air / sin of angle of refraction on glass
=speed of light in air / speed of light in glass
If light is travelling from glass to air gna = sine of angle of incidence on glass / sine of angle of refraction in air
From
principle of reversibility of light we have ang
= 1/ gna
Refractive
Index also called the index of refraction describes how fast light travels
through the material. Refractive index is dimensionless. For a given material,
the refractive index is the ratio between the speed of light in a vacuum and
the speed of light in the medium.
Experimental proof of refraction
(snell’s law)
Refractive Index From A Denser
Medium To A Less Dense Medium
The
depth of a river or a swimming pool always appears shallower than it actually
is. When a glass block is placed on top of an object, e.g a pin or mark on a
piece of paper, the object when view from directly above, appears nearer the
top. This apparent depth is caused by refraction.
n
= real depth / apparent depth
Critical Angle And Total Internal Refraction.
The critical angle is
the angle of incidence in the denser medium when the angle of refraction in the
less dense medium is 90o.
Total internal reflection
is the reflection of an incident ray of light at the interface between the
medium of incidence and another medium of lower refractive index when the angle
of incidence in the denser medium exceeds the critical angle.
Condition for total internal
reflection
I
Light must be travelling from an optically less dense medium. II. The angle of incidence in the denser must
be greater than the critical angle.
n
= 1 / sin c
Example:
If the critical angle for a material is 42°. What is it’s refractive index?
n = 1 / sin c = 1 / sin 42° = 1.49
Application Of Total Internal Reflection
(
binocular, mirage, refractometer optical fibres)
Refraction Through Prism.
When
light rays pass through a rectangular glass block, the incident and the
emergent rays are parallel to each other but the emergent ray is displaced to
one side. There is no deviation or change in the direction of the emergent ray
when compared with the incident ray.
Refractive Index Of Equilateral
Triangle Prism
n =sin 1/2 [A+
Example
A
60o glass prism has a refractive index of 1.63. Calculate the angle of (i)
minimum deviation (ii) refraction of the light passing through the prism at
minimum deviation and (iii) incident of minimum deviation.
Solution
A
= 60oC, n= 1.63
i.
n =
ii.
2r = A; r= 60/2 = 30o
iii. n =
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