Case (i) :
When the incident ray strikes normally at the point of incidence it does not deviate from its path
∠i = 0, ∠r = 0 and d = 0
∠r = 0, d = 0
Case (ii) :
If the refractive indices of two media are equal.
n1 = n2 = n
For Snell's law
n sin i = n sin r
sin i = sin r
∠i = ∠r
Hence the ray passes without any deviation at the boundary.
