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A parallel beam of light is incident on a face of a thin prism having an angle of prism equal to 6°. The angle of minimum deviation is measured to be 3° for this thin prism. What is the refractive index of the material of the thin prism?

Asked in GSEB Board July 2023 · Deviation and minimum deviation

• (1) 0.5
• (2) 2
• (3) 3
• (4) 1.5

Step-by-step solution

Why the other options are wrong

• (1) comes from this is (δₘ)/A=3/6, the deviation per unit prism angle, with the -1 of δₘ=(n-1)A dropped. A refractive index below 1 is impossible in any case.
• (2) comes from this is A/(δₘ)=6/3: the ratio turned over, and again without allowing for the -1.
• (3) comes from this is 1+A/(δₘ), the ratio inverted inside the right formula. A prism that bends light through only half its own angle is a weak one, so n must be near 1.

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