Physics

Problem

Show that, for an optical fibre in air, the numerical aperture

\text{NA}

is given by

\text{NA}=\sqrt{n_{\text{core}}^{2}-n_{\text{clad}}^{2}}

, where

n_{\text{core}}

and

n_{\text{clad}}

are the refractive indices of the core and cladding respectively.