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The Two Classes of Waves

All waves in nature - sound waves, radio waves, any waves - can be classified as one of two types. All waves are either transverse or longitudinal.

Transverse Waves

Consider waves in water. The waves travel horizontally through the water, and finally crash on the shore. But the water itself does not travel in this way. Rather, each 'piece' of water; each water molecule; simply moves up and down again as the wave moves past. The wave itself consists of nothing but these up and down motions of individual particles, 'synchronised' to look like a wave traveling toward the shore on a grand scale.

A transverse wave is simply any wave in which the direction of motion of the particles (or the direction in which the wave oscillates), e.g. vertically up and down in the case of water, is at right angles to the direction in which the whole wave travels ('propagates'). In the case of water, the waves propagate horizontally towards the shore.

Longitudinal Waves

What about sound waves? These are caused by air vibrations which oscillate back and forth parallel to the direction in which the sound wave travels. Such waves are said to be longitudinal. There is no preferred 'up' or 'down' direction for an air particle to move, only backwards and forwards. A speaker cone oscillates in and out to produce the vibrations, and your ear-drum oscillates in and out in response to them.

The two classes of waves

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