“Snell’s law is not valid when light is incident normally on the surface separating the two media”. Why?
When light is incident normally on the surface separating the two media, then it travels undeviated. In this case, angle of incidence, I= angle of refraction, r = 0. Snell’s law is not valid in such case. Because then it implies that 1µ2 = speed of light in medium 1 0 = speed of light in medium 1/speed of light in medium 2 This means that the speed of light in medium-1 has to be 0 (or speed of light in medium-2 must be infinity) and this is not possible.