(a) Using the phenomenon of polarisation, show how the transverse nature of light can be demonstrated. (b) Two polaroids P1 and P2 are placed with their pass axes perpendicular to each other. An unpolarised light of intensity I0 is incident on P1. A third polaroid P3 is kept in between P1 and P2 such that its pass axis makes an angle of 30° with that of P1. Determine the intensity of light transmitted through P1, P2 and P3.



Tie a thick long silk thread about six meters long to a nail on opposite wall in a open ground. Cut a two feet long rectangular slit two inches wide in a thick card board and pass the silk rope through it Hold the cardboard just a feet from your stretched hand and keep the slit vertical . Hold the loose end in your hand in such a way that it does not slip at all out of your grip. Keep the rope a little loose so that you can move it up and down or sideways by quick waving of your hand. 

Now if you keep the slit perfectly vertical and wave your hand also vertically a standing wave l develops ,passes through the slit and goes up to the wall and creates the full return wave as long as you wave your hand. If you turn the slit to a horizontal position and continue to wave your hand up and down in vertical way the wave will stop right at your hand .On the other side there will only be a straight rope. Only if you keep the slit in horizontal direction and start waving your hand also in a sideways fashion agreeing with the horizontal position of slit the full wave upto wall will again start. But the plane of the standing wave will now be horizontal.This stopping of wave is an example of phenomenon of polarisation of transverse vaves. 
Light waves emanating from a source will be in many planes but the the structure of the molecular planes inhibits propagation of light waves in all other directions except only one direction.This is the way polarization occurs.
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