Question

in which case snells law is not applicable

Answer 1

At the point of total internal reflection.

“Snell's law is generally true only for isotropic or specular media (such as glass). In anisotropic media such as some crystals, birefringence may split the refracted ray into two rays, the ordinary or o-ray which follows Snell's law, and the other extraordinaryor e-ray which may not be co-planar with the incident ray”