5.7 Wave Behavior - Refraction

When waves enter a different medium from the one they are moving through, a phenomenon known as refraction occurs, whereby the wave is observed to change its direction. This is a result of the change of speed of the wave in its new medium. For example, let us take the example of light waves and sound waves traveling through water and air.

 Speed in Air Speed in Water Light Waves Faster Slower Sound Waves Slower Faster

While it is the relative differences in the speed of a wave that cause refraction, it can be said that the type of wave (e.g. light or sound) and the two media are the determining factors.

When a wave enters a medium in which travel is more difficult, its speed is decreased. However, the rate at which the waves are coming from the source (the frequency), is not decreased. What happens? As the waves slow down, the distance between each crest, its wavelength, decreases as well. This can be observed even with ocean waves. Open ocean waves can be as large as boats, but those along the shore are sometimes only as big as a swimmer. As waves approach the shore from the deeper sea, they slow down as the bottom gets shallower, and the large waves turn into smaller, closer ones which finally crash on the shore. Of course, the opposite is true as well, and as a wave’s speed is increased due to refraction, so is its wavelength.

The principle of refraction is very straightforward. In general,

1. When a wave travels into a material where it speed is decreased, its path is bent towards the normal, as its wavelength is decreased

2. When a wave travels into a material where its speed is increased, its path bends away from the normal, and its wavelength is increased.