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There is a prism with refractive index equal to √2 and the refracting angle equal to 30°. One of the refracting surface of the prism is polished. A beam of monochromatic light will retrace its path if its angle of incidence over the refracting surface of the prism is

Asked in CBSE AIPMT 1992 · Deviation and minimum deviation

• (1) 0°
• (2) 30°
• (3) 45°
• (4) 60°

Step-by-step solution

Why the other options are wrong

• (1) comes from expecting the beam itself to arrive along the normal; it is the polished face that has to be met normally, which leaves the whole prism angle at the entry face
• (2) comes from is the refracting angle A, which here equals r₁; Snell's law still has to turn that internal angle into the angle outside the glass
• (4) comes from using μ=√3 instead of √2, which gives sin i=(√3)/2

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