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Question
Figure shows a ray of light passing through a prism. If the refracted ray QR is parallel to the base BC, show that (i) r1 = r2 = A/2 and (ii) angle of minimum deviation, Dm = 2i − A.
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Solution
(i) When QR is parallel to the base BC, we have: i = e
\[\Rightarrow r_1 = r_2 = r\]
We know that
\[r_1 + r_2 = A\]
\[ \Rightarrow r + r = A \]
\[ \therefore r = A/2\]
(ii) Also, we have:
A + D= i + e
Substituting, D = Dm and e = i
A + Dm= i + i
∴ Dm=2i - A
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