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,

(ii) angle of minimum deviation, D or D_m = 2i - A.

Answer 1

(i) From given figure, A = r₁+r₂

As the ray QR is parallel to the base BC,

then r₁ = r₂, and i = e

therefore, 2r₁(or 2r₂) = A

⇒ r₁ = r₂ = A/2

(ii) D = (i - r₁ ) + (e - r₂ )

D = (i + e) − (r₁ + r₂)

or D = 2i - A