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.