JEE Advanced · 2008 · 13 Apr 2008 · Paper 1 · Geometrical Optics

Two beams of red and violet colours are made to pass separately through a prism (angle of the prism is

6 0^{\circ}

). In the position of minimum deviation, the angle of refraction will be

(A) $3 0^{\circ}$ for both the colours✓ Correct
(B)greater for the violet colour
(C)greater for the red colour
(D)equal but not 30° for both the colours

Answer: (A)

Solution

At minimum deviation for any wavelength, the ray inside the prism is parallel to the base. This gives

r_{1} = r_{2} = \frac{A}{2}

, where

A

is the prism angle and

r_{1}, r_{2}

are the angles of refraction at the two surfaces. Since

r_{1} + r_{2} = A

, we have

r_{1} = r_{2} = \frac{6 0 ^{\circ}}{2} = 3 0^{\circ}

for both colours.

JEE Advanced · 2008 · 13 Apr 2008 · Paper 1 · Single correct option

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