The change in the direction of the path of light, when it passes from one transparent medium to another transparent medium, is called refraction.
Refraction of light is essentially a surface phenomenon.
Refraction is caused because of the change in the speed of light from one medium to another.
When a ray of light enters a denser medium it is refracted towards normal in such manner than the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, this constant being called the refractive index n.
Refractive Index: - The ratio of the speed of light in a vacuum to the speed of light in the medium is defined as refractive index ‘n’. It is called the absolute refractive index.