Light is incident from a medium into air at two possible angles of incidence (A) 20° and (B) 40°. In the medium light travels 3.0 cm in 0.2 ns. The ray will: (a) suffer total internal reflection in both cases (A) and (B) (b) suffer total internal reflection in case (B) only (c) have partial reflection and partial transmission in case (B) (d) have 100% transmission in case (A)

Answer: (b). Speed in the medium: v = 3×10⁻²0.2×10⁻⁹ = 1.5×10⁸ m/s. n = c/v = 2 ⇒ sin C = 1/2, C = 30°. Case A: 20° 30°, TIR. So the answer is (b) TIR in case (B) only.

Light is incident from a medium into air at two possible angles of… | JEE Main 2013 (9 Apr, Shift 1)

Q50JEE Main 2013 · 9 APR · SHIFT 1Medium

Total Internal Reflection

Light is incident from a medium into air at two possible angles of incidence (A) 20° and (B) 40°. In the medium light travels 3.0 cm in 0.2 ns. The ray will:

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Answer: (b)

Speed in the medium: $v = \frac{3\times10^{-2}}{0.2\times10^{-9}} = 1.5\times10^8$ m/s.
$n = \frac{c}{v} = 2 \Rightarrow \sin C = \frac{1}{2},\ C = 30^\circ$ .
Case A: $20^\circ < 30^\circ$ , partly reflected and partly transmitted. Case B: $40^\circ > 30^\circ$ , TIR.
So the answer is (b) TIR in case (B) only.

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