Explain the difference between a longitudinal and transverse wave.

Imagine we have a wave that is travelling from left to right. This is the wave's direction of motion, or what is sometimes called the direction of "energy propagation". In a transverse wave the particles vibrate perpendicular, that is, at right angles, to the direction of motion; in our wave that means the particles are vibrating up and down as the wave moves along. However, in a longitudinal wave the particles vibrate in the same direction as the wave moves; in our wave that means that they vibrate forwards and backwards as the wave passes. An example of a transverse wave is visible light, whilst a longitudinal wave would be sound.