(i)Numerical Aperture (NA): Numerical Aperture is the light gathering ability or capacity of an optical fiber. More the NA. the more efficient will be fiber. It is also known as figure of merit.

NA is given by equation,

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

Where $\ n_1$ is refractive index of core

$\ n_2$ is refractive index of cladding

(ii) Acceptance angle.(θ): It is the maximum angle made by the light ray with the fiber axis, so that light can propagate through the fiber after total internal reflection.

Relation NA and acceptance angle:

NA = Sin θ

θ = $\ Sin^{-1} NA$