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Physics · 2025 · 2 marks
CBSE 2025 · Region 6 · Set 2 · Q20
A ray of light is incident on face AB of a prism ABC with angle of prism A and emerges out from face AC. The prism is set in the position of minimum…
A ray of light is incident on face AB of a prism ABC with angle of prism A and emerges out from face AC. The prism is set in the position of minimum deviation with angle of deviation $\displaystyle \delta$. Find (i) the angle of incidence and (ii) the angle of refraction on face AB .
Marking-scheme solution
In a prism we know$$A+\delta = i+e \quad \text{and} \quad A = r_1 + r_2$$At minimum deviation$$i = e \quad \text{and} \quad r_1 = r_2 = r$$$$A + D_m = 2i$$$$i = \frac{A + D_m}{2}$$$$A = 2r$$$$r = \frac{A}{2}$$
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CBSE Class 12 Physics past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.