IIT-JEE 2011 · Paper 2 · Physics Q21

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PhysicsSingle correct+3 / -1Question 21 of 60

A light ray traveling in glass medium is incident on glass-air interface at an angle of incidence $\theta$ . The reflected (R) and transmitted (T) intensities, both as function of $\theta$ , are plotted. The correct sketch is

Intensity against θ from 0 to 90°: T falls smoothly from near 100% to zero at 90° while R rises smoothly from near zero to 100% at 90°, the curves crossing midway.

T stays near 100% and R near zero up to a critical angle; there T drops vertically to zero and R jumps to 100%, staying there to 90°. R and T together exceed 100% before the jump.

T slightly below 100% and R small, their sum 100%, up to a critical angle; T then falls vertically to zero and R rises to 100% and stays there up to 90°.

T near 100% falls slowly, then drops sharply at an angle and keeps decreasing to zero at 90°; R rises sharply at that angle and continues increasing to 100% only at 90°.

Answer and worked solution

Official answer: C

Below the critical angle a ray in glass is partly reflected and partly transmitted, with $R+T=100\%$ . R is small and grows slowly with θ, and T falls correspondingly.

At the critical angle total internal reflection sets in: T drops to zero and R jumps to 100% for all larger θ.

Checking the sketches.

(A) has no critical angle.
(B) starts with T = 100% while R > 0, which breaks $R+T=100\%$ .
(D) has transmission beyond the critical angle.
(C) fits.

(C)

Topics:Total internal reflection

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IIT-JEE 2011 · Paper 2, Physics question 21