JEE Main · 2026 · 23 Jan 2026 · Shift 1 · Geometrical Optics

A thin prism with angle

5^{\circ}

of refractive index 1.72 is combined with another prism of refractive index 1.9 to produce dispersion without deviation. The angle of second prism is ______.

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

Answer: (C)

Solution

For dispersion without deviation the net deviation must vanish, so the mean deviations of the two prisms are equal and opposite:

(μ_{1} - 1) A_{1} = (μ_{2} - 1) A_{2}

. Therefore

A_{2} = \frac{( μ _{1} - 1 ) A _{1}}{μ _{2} - 1} = \frac{( 1.72 - 1 ) \times 5 ^{\circ}}{1.9 - 1} = \frac{0.72 \times 5}{0.9} = 4^{\circ}

. The angle of the second prism is

4^{\circ}

JEE Main · 2026 · 23 Jan 2026 · Shift 1 · Single correct option

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