Refraction

Concept: Refraction occurs when light passes from one medium to another and changes its speed and direction. The refractive index quantifies how much light bends in a medium. Lenses use refraction to form images, which may be real or virtual depending on ray convergence. Definition of Refractive Index: The refractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. where, 
= speed of light in vacuum 
= speed of light in the medium 
It also indicates the optical density of the medium. Higher refractive index means greater bending of light. 
Differentiation Between Real and Virtual Images (Lens):