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Geometrical Optics question

2015 · Shift 0 · Q45
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Geometrical Optics question

2015 · Shift 0 · Q45

JEE MainPhysicsGeometrical OpticsMCQ+4 / −1
Monochromatic light is incident on a glass prism of angle AAA. If the refractive index of the material of the prism is μ\muμ, a ray, incident at an angle θ\thetaθ. on the face ABABAB would get transmitted through the face ACACAC of the prism provided : JEE Main 2015 (Offline) Physics - Geometrical Optics Question 211 English
  1. A
    θ>cos⁡−1[μ sin⁡(A+sin⁡−1(1μ)]\theta \gt {\cos ^{ - 1}}\left[ {\mu \,\sin \left( {A + {{\sin }^{ - 1}}} \right.\left( {{1 \over \mu }} \right)} \right]θ>cos−1[μsin(A+sin−1(μ1​)]
  2. B
    θ<cos⁡−1[μ sin⁡(A+sin⁡−1(1μ)]\theta \lt {\cos ^{ - 1}}\left[ {\mu \,\sin \left( {A + {{\sin }^{ - 1}}} \right.\left( {{1 \over \mu }} \right)} \right]θ<cos−1[μsin(A+sin−1(μ1​)]
  3. C
    θ>sin−1[μ sin⁡(A−sin⁡−1(1μ)]\theta \gt si{n^{ - 1}}\left[ {\mu \,\sin \left( {A - {{\sin }^{ - 1}}} \right.\left( {{1 \over \mu }} \right)} \right]θ>sin−1[μsin(A−sin−1(μ1​)]
  4. D
    θ<sin−1[μ sin⁡(A−sin⁡−1(1μ)]\theta \lt si{n^{ - 1}}\left[ {\mu \,\sin \left( {A - {{\sin }^{ - 1}}} \right.\left( {{1 \over \mu }} \right)} \right]θ<sin−1[μsin(A−sin−1(μ1​)]
View written solutionFree

Correct answer: C

  1. Refraction at first face ABABAB

Let the angle of incidence on face ABABAB be θ\thetaθ, and the angle of refraction inside the prism at face ABABAB be r1r_1r1​.

By Snell's law: sin⁡θ=μsin⁡r1\sin \theta = \mu \sin r_1sinθ=μsinr1​ so, r1=sin⁡−1 ⁣(sin⁡θμ)r_1 = \sin^{-1}\!\left(\frac{\sin\theta}{\mu}\right)r1​=sin−1(μsinθ​)

  1. Condition at second face ACACAC

Let the angle of incidence at the second face inside the prism be r2r_2r2​. For a prism, r1+r2=Ar_1 + r_2 = Ar1​+r2​=A Hence, r2=A−r1r_2 = A - r_1r2​=A−r1​

For the ray to emerge from face ACACAC, it must not suffer total internal reflection. Thus, r2≤icr_2 \le i_cr2​≤ic​ where the critical angle ici_cic​ for glass-air interface is ic=sin⁡−1(1μ)i_c = \sin^{-1}\left(\frac{1}{\mu}\right)ic​=sin−1(μ1​)

Therefore, A−r1≤sin⁡−1(1μ)A - r_1 \le \sin^{-1}\left(\frac{1}{\mu}\right)A−r1​≤sin−1(μ1​)

This gives r1≥A−sin⁡−1(1μ)r_1 \ge A - \sin^{-1}\left(\frac{1}{\mu}\right)r1​≥A−sin−1(μ1​)

  1. Convert this into condition on θ\thetaθ

Using sin⁡θ=μsin⁡r1\sin \theta = \mu \sin r_1sinθ=μsinr1​ we get sin⁡θ≥μsin⁡(A−sin⁡−1(1μ))\sin \theta \ge \mu \sin\left(A - \sin^{-1}\left(\frac{1}{\mu}\right)\right)sinθ≥μsin(A−sin−1(μ1​))

Hence, θ≥sin⁡−1[μsin⁡(A−sin⁡−1(1μ))]\theta \ge \sin^{-1}\left[\mu \sin\left(A - \sin^{-1}\left(\frac{1}{\mu}\right)\right)\right]θ≥sin−1[μsin(A−sin−1(μ1​))]

So the ray will be transmitted through face ACACAC provided θ\thetaθ is greater than this limiting value.

Thus the matching option is: θ>sin⁡−1[μsin⁡(A−sin⁡−1(1μ))]\boxed{\theta > \sin^{-1}\left[\mu \sin\left(A - \sin^{-1}\left(\frac{1}{\mu}\right)\right)\right]}θ>sin−1[μsin(A−sin−1(μ1​))]​

  1. Check options
  • A and B involve cos⁡−1\cos^{-1}cos−1, which does not arise here.
  • D has the wrong inequality sign.
  • C matches the derived result.

Therefore, the correct option is C.

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