It can also be derived using the calculus of variations in conjunction with
It can also be derived using the calculus of variations in conjunction with Fermat's principle of least time, or from the Huygens-Fresnel principle. In anisotropic media, only the o-ray and not the e-ray follows this law, which can also be used to find the critical angle of total internal reflection. FTP, name this law of optics that says the ratio of the sines of the indcidence angles of a ray is equal to the ratio of the refractive indices of the medium.
It can also be derived using the calculus of variations in conjunction with Fermat's principle of least time, or from the Huygens-Fresnel principle.
{Snell’s} Law