When it enters at right-angles to the surface. The sine of the angle of refraction is proportional to the sine of the angle of incidence, so if the angle of incidence is zero, so too is the angle of refraction.
When it enters at right-angles to the surface. The sine of the angle of refraction is proportional to the sine of the angle of incidence, so if the angle of incidence is zero, so too is the angle of refraction.