CBSE Class 10 Answered

Refraction is the concept which applies to any wave (Light, sound, whatever) when that wave is passing between materials of different densities, and is intersecting them at an angle. Waves of any time move more slowly through more dense material - with light that is measured by the material's refraction index.

The reason for the bend is simply that every wave has a very slight thickness to it, and when approaching a surface at an angle, part of it will hit first. In your case, the part that hits first will slow down, then the middle of the wave will hit a split second later, and slow down, then the outside of the wave will hit last, and slow down as well. Because of the thickness and the progressive slowing of the wave from one side to the other, it will "bend" the direction of the light.