Drop a stone in a pond and a leaf floating nearby bobs up and down as the ripples pass, then settles where it started. The ripples carried energy across the pond; the water itself did not travel with them.

That is true of every wave: it transfers energy (and often information) from one place to another without carrying the medium along.

There are two kinds, told apart by the way the vibrations (oscillations) go:

• Transverse: the vibrations are at right angles to the direction the energy travels. Ripples on water, waves on a string, and every electromagnetic wave (light, radio, X-rays) are transverse.
• Longitudinal: the vibrations are along the same direction the energy travels. Sound is longitudinal. The medium bunches up into compressions and spreads out into rarefactions.

C marks a compression and R a rarefaction. One wavelength is the distance from one compression to the next.

A graph of displacement against distance is a snapshot of the whole wave at one instant.

• Amplitude: the biggest displacement from the rest position, measured from the middle line to a crest. Here it is 2 cm.
• Wavelength (λ): the distance from a point on one wave to the same point on the next, such as crest to crest. Here it is 8 cm.

Amplitude is middle to crest, not trough to crest. If you measured 4 cm on this graph, halve it.

• Frequency (f): how many waves pass a point each second, in hertz (Hz). 1 Hz is one wave per second.
• Period (T): the time for one complete wave to pass, in seconds.

period = 1 ÷ frequency
T = 1 / f

T in seconds (s), f in hertz (Hz). AQA prints this on its Physics equation sheet; learn it anyway, since it is one line and works both ways (f = 1/T).

1. A loudspeaker vibrates at 250 Hz
2. T = 1 ÷ 250 = 0.004 s
3. 0.004 s × 1000 = 4 ms

Period in milliseconds goes back into seconds (divide by 1000) before you use f = 1/T.