Question

what is refraction by a glass slab

Answer 1

when the ray of light is passing fromglass to air that is from a denser. medium to a rarer medium. ... but therefracted ray emerging out of theglass slab is parallel to the incident ray. this means that the refracted ray emergent.

Answer 2

When a light enters a medium 1 say glass ,which is denser than air,it travels with a lower speed .But when the light Ray enters another medium say air,the refracted light ray shows a deviation from it's path and slightly bends due to it's lower density.The angle of reflection changes when the light Ray travels to air.
This phenomena of bending of light when light Ray travels from one medium to another is called refraction of light