decreases and at some point becomes less than the critical angle. When equals the critical angle, equals . The maximum launching angle then corresponds to .
4.
Let us apply Snell's law at the launching point and at the core-cladding interface for the maximum launching angle
. For this case let and
we then have
(since )
now,
So the sine of the maximum angle at which the ray will be guided inside the fiber is given by square root of the difference of squares of the refractive indices of the core and cladding. The quantity is called the NUMERICAL APERTURE of an optical fiber. The NA is a measure of the power launching efficiently of an optical fiber.
, 
decreases and at some point becomes less than the critical angle. When 
equals the critical angle, 
equals 
. The maximum launching angle then corresponds to 
. 
. For this case let 
and 
(since 
) 
is called the NUMERICAL APERTURE of an optical fiber. The NA is a measure of the power launching efficiently of an optical fiber. 
