Refraction happens when a wave crosses a boundary between Medium 1 and Medium 2, the wave changes direction because it changes velocity. Frequency remains constant. Velocity changes as a result of wavelength change. 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.
Introduction to Refraction
