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(a) (i) How does an unpolarised light incident on a polaroid get polarised ? (ii) Describe briefly, with the help of a necessary diagram, the polarisation of light by reflection from a transparent medium. (b) Two polaroids 'A' and 'B' are kept in crossed position. how should a third polaroid 'C' be placed between them so that the intensity of polarised light transmitted by polaroid B reduces to 1/8th of the intensity of unpolarised light incidennt on A? |
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Answer» SOLUTION :(b) Let TWO POLAROIDS A and B are KEPT in crossed position i.e., angle between their pass axes is `90^(@)`. Let a THIRD polaroid C be placed between them at an angle `theta` from polaroid A and angle `(90^(@)-theta)` from polaroid B. Let intensity of unpolarised light incident on polaroid A be `I_(0)`. Then intensity of light emerging from polaroid A, `I_(1)=(I_(0))/(2)` `therefore` Intensity of light emerging from polaroid A, `I_(1)=(I_(0))/(2)` `therefore` Intensity of light emerging from polaroid C, `I_(2)=(I_(0))/(2)cos^(2)theta` and final intensity of light emerging from polaroid B, `I_(3)=I_(2)cos^(2)(90^(@)-theta)` `therefore I_(3)=I_(2)sin^(2)theta=(I_(0))/(2)cos^(2)thetasin^(2)theta=(I_(0))/(8)sin^(2)(2theta)` As per question `I_(3)=(I_(0))/(8),` hence we have, `(I_(0))/(8)=(I_(0))/(8)sin^(2)(2theta)` `implies sin^(2)(2theta)=1implies2theta=90^(@) implies theta=45^(@) ` or `(pi)/(4)`. |
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