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A ray of light travelling in impure water is incident on a glass plate immersed in it. When the angle of incidence is 51° the reflected ray is totally plane polarised. The refractive index of the impure water is 1.4. The refractive index of the glass should be (tan 51°=1.235)

Asked in GUJCET 2018 · Brewster's law and polarisation by reflection

• (1) 1.64
• (2) 1.34
• (3) 1.53
• (4) 1.73

Step-by-step solution

Why the other options are wrong

• (1) Comes from using 1.33, the index of pure water, in place of the 1.4 given for this impure water: 1.33×1.235=1.64.
• (2) The index of water handed back. The question asks for the glass, and μ_g=1.4×1.235=1.73.
• (3) 1.53 is the textbook index of ordinary glass recalled from memory instead of worked out from the data printed.

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