A beam of light consisting of red, green and blue colours is incident on a right angled prism. The refractive index of the material of the prism for the above red, green and blue wavelengths are 1.39, 1.44 and 1.47 respectively. The prism will (a) not separate the three colours at all (b) separate the red colour part from the green and blue colours (c) separate the blue colour part from the red and green colours (d) separate all the three colours from one another

Answer: (b). Rays enter face AB normally and strike face AC at an angle of incidence 45°. TIR at AC needs sin 45° ≥ 1/μ, i.e. μ ≥ √(2) ≈ 1.414. Red ( 1.39 < 1.414) comes out of AC; green ( 1.44) and blue ( 1.47) are totally reflected. So the answer is (b) the red part is separated from green and blue.

A beam of light consisting of red, green and blue colours is incident… | NEET 2015 (AIPMT)

Q47NEET 2015 · AIPMTMedium

Total Internal Reflection

A beam of light consisting of red, green and blue colours is incident on a right angled prism. The refractive index of the material of the prism for the above red, green and blue wavelengths are 1.39, 1.44 and 1.47 respectively.

The prism will

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Answer: (b)

Rays enter face AB normally and strike face AC at an angle of incidence $45^\circ$ .
TIR at AC needs $\sin45^\circ \ge \frac{1}{\mu}$ , i.e. $\mu \ge \sqrt{2} \approx 1.414$ .
Red ( $1.39 < 1.414$ ) comes out of AC; green ( $1.44$ ) and blue ( $1.47$ ) are totally reflected.
So the answer is (b) the red part is separated from green and blue.

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