When a wave passes from a medium in which it travels faster to a medium in which it travels slower, it is refracted toward the normal (the perpendicular to the boundary between the two media) to the boundary surface. The angle of refraction (the angle between the refracted wave and the normal) is smaller than the angle of incidence (the angle between the incident wave and the normal). Conversely, when a wave passes from a medium in which it travels slower to a medium in which it travels faster, it is refracted away from the normal to the boundary surface. The angle of refraction is then greater than the angle of incidence.
The speed of a wave in a medium is determined by the properties of the medium, including its density and elasticity. In general, waves travel faster in denser and more elastic media. For example, waves travel faster in glass than in air and in water than in air.
Refraction of waves is a common phenomenon that has a wide range of applications, including in the fields of optics, acoustics, and seismology.
