Polarisation
Polarisation
A light wave is an electromagnetic wave that travels
through the vacuum of outer space. Light waves are produced by vibrating
electric charges.
Ordinary light has electric vectors in all possible directions
in a plane perpendicular to the direction of propagation of light. Hence it is
called unpolarised light.
It can be considered a combination of vertical and
horizontal components. Average of half its vibrations are in a horizontal plane
and half of its vibrations are in a vertical plane.
The phenomenon of restructuring of electric vectors of
light into a single direction is called polarisation.
When ordinary light is passed through a tourmaline,
calcite or quartz crystal the transmitted light has electric vectors in a
particular direction parallel to the axis of crystal. This light is called
plane polarised
light.
A plane containing the vibrations of polarised light
is called plane
of vibration.
A plane perpendicular to the plane of vibration is
called plane
of polarisation.
Polarisation can take place only in transverse waves.
Polarisation by transmission
When unpolarized light is
transmitted through a Polaroid
filter, it emerges with one-half the intensity and with vibrations in a
single plane; it emerges as polarized light.
Polarisation by scattering
When we look at the light scattered by the molecules
at 90o to
direction of incident light, we see only the perpendicular component of the
light. The radiation scattered by the molecules is polarized perpendicular to
the plane of the figure.
Polarisation by reflection - Brewster’s law
When unpolarised light is incident at an angle of polarisation (iB) on
the interface separating air from a medium of refractive index n, then
reflected light becomes fully polarised, provided
n = tan iB
Refractive index
n =
tan iB = 
where, C = critical
angle.
If angle of refraction
is r then,
iB + r = 90°
Law of Malus
When a beam of
completely plane polarised light is incident on an analyser, the intensity of
transmitted light from analyser is directly proportional to the square of the
cosine of the angle between plane of transmission of analyser and polariser,
i.e.,
I ∝ cos2 θ
When ordinary light is
incident on a polariser the intensity of transmitted light is half of the
intensity of incident light.
When a polariser and analyser
are perpendicular to each other, then intensity of transmitted light from
analyser becomes 0.
Polaroid
It is a polarising film
mounted between two glass plates.
Uses of polaroid
i.
It
is used to produce polarised light.
ii.
A polaroid is used to avoid glare of light in spectacles. Polaroids are used in sun glasses. They protect the eyes
from glare.
iii.
The polaroids are used in window panes
of a trains and an aeroplane. They help to control the light entering through
the window.
iv.
The
pictures taken by a stereoscopic camera, when seen with the help of polarized
spectacles, create three dimensional effect.
v.
The
windshield of an automobile is made of polaroid. Such
a wind shield protects the eyes of the driver of the automobile from the
dazzling light of the approaching vehicles.
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