Refraction is the bending of a light wave when it enters a different medium from the previous one. The change in the speed of light when it crosses one medium and enters another causes the bending of light at the interface.
The reduction in the speed of light in the new medium can be determined by the refractive index, which equals the ratio of the speed of light in vacuum to the speed of light in the given medium. Light travels slowly in a medium that has a high density, because it interacts with the particles of the medium. Snell's Law explains the relation between the angle of the incident light ray from the normal and the angle of the refracted light ray from the normal.