Question

In an optical fibre with the core and cladding having refractive index $n_1$ and $n_2$ respectively, $(n_1>n_2)$, the numerical aperture is _____ when the light enters the fibre in air.

• A. $NA=\frac{\sqrt{n_1^2-n_2^2}}{n_2}$
• B. $NA=\frac{\sqrt{n_1^2-n_2^2}}{n_1}$
• C. $NA=\sqrt{n_1^2-n_2^2}$
  
• D. None of these

Answer 1

The correct option is C $NA=\sqrt{n_1^2-n_2^2}$


$NA=\sqrt{n_1^2-n_2^2}$

where,
$n_1\Rightarrow \text{R.I of core}$

$n_2\Rightarrow \text{R.I of cladding}$