Refraction occurs when the speed of the wave changes as it travels from one medium to a        different medium.                                                 As the incident ray enters the glass, it bends towards the normal, this is because it slows down        as glass is more dense than air.     As the refracted ray leaves the glass to air, it bends away from the normal, this is because it          speeds up as air is less dense than glass.     Water waves do refract too. They travel slower in shallow water but speed up in deep water.