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Ordinary light is incident on a glass slab at the polarising angle and suffers a deviation of 22° on refraction. The value of the angle of refraction is

Asked in GSEB Board July 2015 · Brewster's law and polarisation by reflection

• (1) 74°
• (2) 22°
• (3) 90°
• (4) 34°

Step-by-step solution

Why the other options are wrong

• (1) No route gives 74°: the pair of conditions iₚ+r=90° and iₚ-r=22° fixes iₚ=56° and r=34°. An angle of refraction bigger than the angle of incidence would also mean the light had entered a rarer medium, not glass.
• (2) Hands back the deviation as the answer. The deviation is the difference iₚ-r between the two directions, not the angle of refraction itself.
• (3) Confuses the 90° between the reflected and the refracted rays -- which is what makes this the polarising angle -- with the angle of refraction.

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