(a) A ray of light is incident on one face of a triangular glass prism of refractive index $n$ and is refracted out from the opposite face. Deduce the expression for refractive index of glass prism in terms of the angle of minimum deviation and angle of the prism. Write the condition on $n$ for refraction to take place on the opposite face. OR (b) State Huygens principle. Using it, draw a diagram and discuss the case of refraction of plane wave of light from a rarer medium to a denser medium at their plane interface. Hence derive Snell's law.