Polarisation - Light is a transverse wave consisting of vibrations normal to the direction of propagation. The lack of symmetry of vibration around the direction of wave propagation is called polarisation.
Polarisation is the only phenomenon which proves that light is a transverse wave. On the other hand the sound waves are longitudinal waves and do not show polarisation.
Unpolarised light - If the vibrations of electric vector can take place in all possible directions, perpendicular to direction of propagation then the light is said to be unpolarised light.
e.g. light from a ordinary source of light is unpolarised.
The angle between the plane of polarisation and plane of the device which produces polarised light is called polariser and the one which detects polarised light is called analyser.
Polarisation of Light :
Light is a form of energy which enables us to see the objects from which it comes.
Light waves are electromagnetic in nature can travel with a velocity of 3 x 108 m/sec. in free space.
Light waves show a number of properties such as interference, diffraction, polarsation, doppler effect, reflection, refraction and dispersion etc.
Polarisation of light is one of the unique and most significant properties shown by light waves.
In this project report, we shall study in details about the polarisation of light.
Theory of Polarised Light
Plane Polarised Light - A plane polarised light is that in which vibrations are confined only to one direction.
If the vibrations are parallel to the plane of paper, they are represented by arrows. If they are along a straight line perpendicular to the plane of the paper then they are represented by dots.
Vibrations parallel to the plane of paper.
Vibrations perpendicular to the plane of paper.
The plane in which vibrations of polarised light are taking place is called plane of vibration.
The plane containing the direction of propagation of light and perpendicular to the plane of vibration is called the plane of polarisation.
Light Waves are Transverse in Nature
The polarisation of light is shown by a tourmaline crystal (aluminium boron silicate). It absorbs all the vibrations of the incident unpolarised light except the one parallel to its axis. It, therefore, polarises light.
When ordinary light is incident normally on a pair of parallel tourmaline crystal plates P1 and P2 cut parallel to their crystallographic axis the emergent light shows a variation in intensity as P2 is rotated. The intensity is maximum when the axis of P2 is parallel to that of P1 and minimum when at right angles.
This observation clearly verifies that vibrations in light waves are transverse. If light waves were longitudinal, then intensity of light emerging from P2 would have always been the same i.e. there would have been no effect on the intensity of light wave due to the rotation of crystal P2 relative to P1.
To Detect Plane Polarised Light
When unpolarised light is seen through a single crystal (polaroid) intensity of transmitted light decreases, on account of polarisation, on rotating crystal, intensity of polarised light doesn't change.
Actually a combination of polariser and analyser helps in detection of plane polarised light.
Methods of Production of Plane Polarised Light
(a) By reflection of light (Polarisation by reflection)- Brewster found that the light is totally polarised when reflected at a transparent medium (as glass), if incident at a particular angle ip given by Brewster’s law.
(b) Polarization by Scattering - When a beam of white light is pass through a medium containing particles whose size is of the order of wavelength of light. Then the beam gets scattered. When the scattered light is seen in a direction perpendicular to the direction of incidence, it is found to be plane polarised (as detected by analyser). This is called polarization by scattering.
(c) By Dichroism - Some doubly- refracting crystals have the property of absorbing strongly one of the two refracted rays, while allowing the other to emerge with little loss. This selective absorption by the crystals is known as dichroism. The best example of such a crystal is tourmaline.
Information about Polaroids
Polaroids - Polaroid is based on the phenomenon of dichroism. It is a big polarising film mounted between two glass plates and is used to obtain plane polarised light. To prepare this film, ultra-microscopic crystals of herpathite (lodosulphate of Quinine) are spread by a special device in a thin sheet of nitro-cellulose in such a way that the optic axes of all the crystals are parallel. Hence, when a beam of unpolarised light passes through the polaroid film, the emerging light is plane-polarised.
Uses of Polaroids -
a) Polaroids are used in the laboratory to produce and analyse plane Polarised light.
b) They are used in sun glasses to cut off the light reflected from horizontal surfaces such as wet roads, polished tables etc.
c) Polaroids are used in motor cars to avoid the dazzling light of a car approaching from opposite directions.
d) Polaroid glasses are used for viewing three dimensional pictures.
e) Polaroids are used in photographic camera as filters. These filters eliminate the glare of the reflected light.
Malus Law-When a completely plane Polarised light beam is incident on an analyser, the intensity of the emergent light varies as the square of the cosine of the angle between axes of polariser and analyser. It is known as Malus Law.
i. e I µ Cos2 q
or, I = I0 Cos2q
Ifq = 0 i.e. the polariser and analyser are parallel
then I = I0 (Maximum)
and ifq = 900 i.e. the polariser and analyser are perpendicular.
then I = 0 (Minimum)
There are certain substances which have property to rotate the plane polarised light's plane of vibration when passed through them, this is called optical activity.
Such substances are called optically active substances.
Two Categories of Substances
(i) Dextrorotatory (Right Handed)
Substances that rotate light in clock wise direction are called dextrorotatory.
(ii) Laevorotatory (Left Handed)
Substances that rotate light in anti-clock wise direction are called laevorotatory.
Dependence of Angle of Rotation :
(i) Thickness of crystal.
(ii) Density of crystal.
(iii) Wavelength of light used.
(iv) Temperature of the solution.
1. Teacher's Notes.