Numerical Aperture (NA) of any optical system is a measure of how much light can be collected by the optical system. It is the product of the refractive index of the incident medium and the sine of the maximum ray angle.

 NA = n_i.sinθ_max, n_i for air is 1, hence NA = sinθ_max

For a step-index fiber, as in the present case, the numerical aperture is given by

 NA= (n_core² –n_cladding²)^½

For very small difference in refractive indices the equation reduces to

 NA=n_core (2Δ)^½ where Δ is the fractional difference in refractive indices.

The experimenter may refer to the specifications of the PMMA fiber given in Appendix I and record the manufacturer’s NA, n_cladding and n_core, and θ