JEE Main · 2019 · 11 Jan 2019 · Shift 2 · Geometrical Optics

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A monochromatic light is incident at a certain angle on an equilateral triangular prism and suffers minimum deviation. If the refractive index of the material of the prism is

3

, then the angle of incidence

(A) $9 0^{\circ}$
(B) $3 0^{\circ}$
(C) $6 0^{\circ}$ ✓ Correct
(D) $4 5^{\circ}$

Answer: (C)

Solution

At minimum deviation through an equilateral prism (

A = 6 0^{\circ}

) the ray passes symmetrically, so each refraction angle is

r = \frac{A}{2} = 3 0^{\circ}

. Snell's law at the first face gives

sin i = μ sin r = 3 \times sin 3 0^{\circ} = 3 \times \frac{1}{2} = \frac{3}{2}

, so

i = 6 0^{\circ}

JEE Main · 2019 · 11 Jan 2019 · Shift 2 · Single correct option

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