Refraction of Waves

About this topic

Refraction is the bending of a wave as it changes speed on entering a new medium. Frequency stays constant, so wavelength changes with speed. A wave slowing down bends toward the normal.

Water waves bend as they enter shallow water and sound bends upward in cool air above warm ground, which is why sound carries well across lakes in the evening. The relation v = fλ means that a slower wave has a shorter wavelength, bunching the wavefronts closer together. For light, the same bending forms the basis of lenses, glasses and prisms.

Worked example: waves moving from deep water into shallow water at an angle slow down, wavelength shrinks, and the wave direction turns toward the normal. That is why waves near a beach tend to arrive nearly parallel to the shore, even if they began at an angle far out. A straw in a glass looks bent at the surface for the same reason.

Seismic waves refract inside Earth, which helped scientists map the core. A common misconception is that the frequency changes when a wave enters a new medium; it does not, because the number of waves arriving each second must equal the number leaving. Another is that a wave bends because it is pushed; it bends because one side slows first.