What should be the maximum acceptance angle at the air-core interface of an optical fibre if $n_1$ and $n_2$ are the refractive indices of the core and the cladding, respectively?

• A

  $\sin ^{-1}\left(n_2 / n_1\right)$
• Correct answer
  B

  $\sin ^{-1} \sqrt{n_1^2-n_2^2}$
• C

  $\left[\tan ^{-1} \frac{n_2}{n_1}\right]$
• D

  $\left[\tan ^{-1} \frac{n_1}{n_2}\right]$

Hence, the correct answer is option: 'B'.