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, due to which the direction of light changes, and thus the bending of light occurs.

It does not bend if it enters the glass at right angles (90°) because no refraction occurs in this case, as 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. Here, in this case, the angle of incidence is zero, and the angle of refraction is also zero.

Explanation

The angle of refraction or the amount of bending of light depends on the refractive index  of the two media.

1. When light passes through a low refractive index, like air (here water) (faster medium or optically rarer medium) to a high refractive index, like glass (slower medium or optically denser medium), its speed decreases and bends towards the normal. 

2. When light passes through a high refractive index, like glass (slower medium or optically denser medium) to a low refractive index, like air (here water) (faster medium or optically rarer medium) its speed increases and bends away from the normal. 

3. However, if the light ray hits the glass block at right angles (at 90°) to the surface, then the light ray will not bend.