Fermat's principle of least time.
This equation can be used to predict the maximum acceptance angle in terms of the properties of the core and the cladding materials. A derivation of this equation applies the appropriate boundary conditions to Maxwell's equations to show that the parallel component of the wavevector is constant, while a simpler geometric derivation obtains it directly from (*) Fermat's principle of least time. This equation is used to calculate the critical angle above which total internal reflection occurs. This equation states that the product of the sine of the angle from the normal and the index of refraction is constant. For 10 points, name this law named for a Dutch scientist that describes how light refracts as it encounters an interface.
This equation can be used to predict the maximum acceptance angle in terms of the properties of the core and the cladding materials.
Snell's law 