What happens when light enters from
(a) denser medium to rarer medium
(b) rarer medium to denser medium

Hint: Light is a form of energy that can be detected by the human eye. Light travels in the form of waves. It is one of the important sources of energy. It is composed of photons, which are massless particles. Thus we can say that light does not have any mass.

Complete answer:
The medium in which the speed of the light is less is called a denser and the medium in which the speed of the light is more is called a rarer medium. Water is the example for denser medium and air is the example for rarer medium.
The medium which has the low refractive index will have the highest optical density and vice-versa. Thus light will travel slowly in the denser medium which indicates a higher index of refraction. Thus light waves travel faster through air and vacuum and slowly through others like glass and water.
Thus when the light travels from a denser medium to a rarer medium, it bends away from the normal due to the increase in the speed. Similarly, when the light travels from a rarer medium to a denser medium, it moves towards the normal due to the decrease in the speed.

Note: 1. The substance that transfers the energy from one place to another or from one surface to another surface is called the medium. In simple terms, it acts as a carrier. The medium can transfer any form of heat, light, energy.
2. Light behaves as a ray, wave, and particle. When light travels from one medium to another medium, it gets absorbed or gets reflected back or else passes through it.