[10.2.11] The acceptance half-angle is set by the refractive indices of the core and cladding of fiber optics. It is expressed in terms of the numerical aperture (NA).The numerical aperture is given as:

NA = n_{0}\sin \theta _{max} ,

where:

θmax is the greatest angle from the optical axis of a circular fiber where the external meridional plane ray will undergo total internal reflection within the fiber

n0 is the refractive index of the medium outside the fiber (for air, n0 = 1)

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