Disappearing coin

Refraction is the bending of a wave when it enters a medium where its speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the index of refraction (n=c/v c-velocity of light in vacuum v-velocity of light in medium) of the two media and is described quantitatively by Snell's Law.

Snell's law gives the relationship between angles of incidence and refraction for a wave impinging on an interface between two media with different index of refraction. The law follows from the boundary condition that a wave be continuous across a boundary, which requires that the phase of the wave be constant on any given plane.