What is an optical rarer medium?

Hint: As compared to glass and water air is an optically rare medium. The velocity of the light is faster in an optically rare medium than the velocity of light from an optically rare medium. Based on the velocity of light in the medium the optically rarer or denser medium is categorised.

Complete step-by-step solution:
A medium in which the velocity of light is more is called optically rare medium. The light changes its direction when it enters an optically dense medium from an optically dense medium and is towards the normal to the surface is refracted. The composition of air here, we have assumed as constant and hence in the medium the speed of the light is constant. The electromagnetic waves pass through a media known as optical media. Due to its different optical density every optical medium has its own refractive index. Refraction Index refers to the degree to which a medium manages to slow down the wave that is transmitted through.

Note: The ray bends more towards the normal when a wave passes from an optically rare to an optically denser medium. For eg- in air and water, the air is rare and water is a denser medium. Here, in an optically rare medium the speed of the light is more.