If angle of incidence is 90 what is the angle of refraction?

Question

If angle of incidence is 90 what is the angle of refraction?

Answer 1

Trick question! If AOI is exactly equal to 90 degrees, then the rays never hit the surface!  So there is no refraction, because there is no interaction.

You might be tempted to apply snell's law:

n1sint1 = n2sint2

and so

sin(theta2) = n1/n2 when theta1 = 90degrees.  In which case you'd find that, say for air to glass, theta2 = 41.8 degrees.  But, snell's law only really applies for theta<90degrees.  For instance, it would not make sense to talk about refraction with an AOI of 100 degrees (which would mean the light was travelling away from the surface, so never gets refracted!)