CBSE Class 12 Physics 2018 Delhi Set 1 Paper

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Question : 15 of 26
 
Marks: +1, -0
(a) Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface.
(b) The figure shows a ray of light falling normally on the face AB of an equilateral glass prism having refractive index 32, placed in water of refractive index 43. Will this ray suffer total internal reflection on striking the face AC ? Justify your answer.
Solution:
(a) Diagram
Polarisation by reflection
(b) Justification
Writing yes/no
(a) The diagram, showing polarisation by reflection is as shown.
[Here the reflected and refracted rays are at right angle to each other.]

∴r=(π2−iB)
∴µ=(siniBsinr=taniB)
Thus light gets totally polarised by reflection when it is incident at an angle iB (Brewster's angle), where iB=tan−1µ
(b) The angle of incidence, of the ray, on striking the face AC is i=60∘ (as from figure)
Also, relative refractive index of glass, with respect to the surrounding water, is
∴µr=3243=98
Also sini=sin60∘=32=1.7322=0.866

For total internal reflection, the required critical angle, in this case, is given by
sinic=1µ=89≃0−89
∴i<ic
Hence the ray would not suffer total internal reflection on striking the face AC
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