Why does a beam of light when it enters glass at an angle? Why does it not bend if it inters the glass at right angles?
When a beam of light enters glass at an angle, the speed changes and therefore the direction of light changes; i.e., bending of light occurs.
When a beam of light falls at right angles to the surface of glass, all parts of light waves reach the glass at the same time, enter the glass at the same time and hence slow down at the same time. Due to this no change in direction of light takes place; i.e., bending of light does not take place.
Lakhmir Physics 2013