Snell's law states that
u(constant of refraction) = sini/sinr
If a ray of light is passing through a glass slab through its normal, then sin i = 0 and thus the constant of refraction is not mantained.
But as the definition of snell's law suggests that this law is applicable to light travelling obliquely. If this is so, I have a question:
Refraction occurs due to the change in velocity of light in 2 different mediums. This change in velocity occurs as the atoms in a particular media has a specific frequency and if the light wave has a different frequecy, only then will it let the light pass through. But in this process, the light is refracted at some angle. BUT WHY DOES THIS REFRACTION NOT OCCUR WHEN LIGHT TRAVELS THROUGH THE NORMAL?
(I hope you understand the question. If not, pls tell me where I am not clear, i'll try to explain in a better way)
The key to answering the question is too study the Snell's Law more carefully. The Snell's Law states that the ratio of the sines of the incident and the refracted angles is equal to reverse ratio of their respective refractive indices. Mathematically, it can be represented as
here n1 is the refractive index of the first medium, n2 is the refractive index of the second medium (glass slab), i is the angle of incidence and r is the angle of refraction.
If the light ray passes the glass slab through its normal (or even parallel to it) then from the above equation n2 = 0 for any value of n1 and sinr approaches infinity, the Snell's Law collapses. Refraction, thus will not take place under such conditions.
This is why Snell's Law is only applicable for oblique incidence angles.