Question

What happens if a incident ray passes straight along the normal through the second medium

Answer 1

When a ray of light falls normally to the surface of a glass slab (or any other medium for that matter) it makes an angle of 0 degree with respect to its normal. So, i = 0, now in this case,

nsini = n'sinr [n and n' are respective refractive indices]

so, n'sinr = 0

as n cannot be zero so sinr = 0 or angle of refraction (with normal) r = 0.

Thus, no refraction takes place and the ray of light emerges through the medium (of n') unrefracted/undeviated.

This is why it passes straight through.