A ray of light incident on the face A B of an isosceles triangular prism makes an angle of incidence (i) and deviates by angle \beta as shown in the figure. Show that in the position of minimum deviation \angle \beta=\angle \alpha. Also find out the condition when the refracted ray Q R suffers total internal reflection.

CBSE Class 12 Physics 2019 Outside Delhi Set 2 Paper

Question : 8

Total: 9

A ray of light incident on the face AB of an isosceles triangular prism makes an angle of incidence (i) and deviates by angle β as shown in the figure. Show that in the position of minimum deviation ∠β=∠α. Also find out the condition when the refracted ray QR suffers total internal reflection.

Solution:

Proving α=β
Finding ic
For minimum deviation,
‌r1+r2=A;‌‌r1=r2
‌(90∘−β)+(90∘−β)=A
‌180∘−2β=A
‌2β=180∘−A
‌2β=2α
‌β=α
‌r1+r2=A
‌r1+ic=A
‌ic=A−r1
‌ic=A−(90∘−β)

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