JEE Physics 2026 Question with Solution

Given: A thin prism has angle $A_1 = 5^\circ$ and refractive index $\mu_1 = 1.72$. It is combined with another prism of refractive index $\mu_2 = 1.9$.

Find: The angle $A_2$ of the second prism for dispersion without deviation.

For dispersion without deviation, the net deviation produced by the combination of the two prisms must be zero.

For a thin prism,

$$\delta = (\mu - 1)A$$

So the condition is

$$\delta_1 + \delta_2 = 0 \implies (\mu_1 - 1)A_1 = (\mu_2 - 1)A_2$$

Substituting the given values,

$$(1.72 - 1) \times 5^\circ = (1.9 - 1) \times A_2$$ $$0.72 \times 5 = 0.9 \times A_2$$ $$3.6 = 0.9 \times A_2$$ $$A_2 = \frac{3.6}{0.9} = 4^\circ$$

Therefore, the angle of the second prism is $4^\circ$. The correct option is C.