where
:? is the angle the ray makes with the fiber axis, before entering the fiber,
:n₀ is the refractive index along the central axis (core) of the fiber, and
:n_c is the refractive index of the cladding.

The quantity is the numerical aperture of the fiber. The quantity is sometimes called the total acceptance angle of the fiber.

This result can be derived from Snell's law

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