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JEE Advanced2015PhysicsRay OpticsActual

Paragraph: Light guidance in an optical fiber can be understood by considering a structure comprising of thin solid glass cylinder of refractive index n₁ surrounded by a medium of lower refractive index n₂ . The light guidance in the structure takes place due to successive total internal reflections at the interface of the media n₁ and n₂ as shown in the figure. All rays with the angle of incidence i less than a part

Options

  1. AN A 1 N A 2 N A 1 + N A 2
  2. BN A 1 + N A 2
  3. CN A 1
  4. DN A 2

Correct answer

D. N A 2

Step-by-step solution

It is given that N A 2 < N A 1 ⇒ i m 2 < i m 1 Hence if the combination can be placed both ways i.e. 1 s t structure & then 2 n d structure and then reversed also, then the condition of TIR is satisfied for lower i m then it can be satisfied for all other less angles as well. Hence N A 2 will be the numerical aperture of the combined structure.

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