Transverse and longitudinal waves — AQA GCSE Physics

Ripples on water are the standard transverse example. The surface of the water moves up and down about its undisturbed level while the ripple itself travels horizontally outwards, so the oscillations are perpendicular to the direction of energy transfer. A floating cork or a duck on a pond shows this directly: it bobs up and down as each ripple passes, but it does not get carried to the far bank, because the water is not travelling with the wave. Only the energy moves across the pond. The wavelength is measured from one crest to the next crest, and the amplitude from the undisturbed level to a crest, not from a crest down to a trough. In the laboratory the same wave is studied in a ripple tank, where the shadows of the ripples are projected onto paper beneath the tank.

In a longitudinal wave the particles oscillate backwards and forwards along the same line that the wave travels, so they bunch up in some places and spread out in others. A region where the particles are pushed closer together, and the pressure is higher than normal, is a compression. A region where they are further apart, with lower than normal pressure, is a rarefaction. The pattern moves forward while each particle stays around its own average position. The wavelength is the distance from one compression to the next compression, or from one rarefaction to the next, which is why a diagram must show at least two of them before it can be measured. In air a compression is a region of slightly raised pressure, and it is that pressure variation which a microphone or an eardrum detects.

Sound is a mechanical wave carried by the vibration of the particles of a medium. A loudspeaker cone moving outwards pushes the air molecules in front of it together, creating a compression; as it moves back it leaves them further apart, creating a rarefaction. The molecules oscillate backwards and forwards along the direction the sound travels, so the wave is longitudinal. Because it needs particles, sound cannot cross a vacuum, unlike light. Sound in air travels at roughly 330 m/s, around a million times slower than light, which is why thunder arrives seconds after the flash. Sound travels faster in solids and liquids, where the particles are closer together. The greater the amplitude of the vibration the louder the sound, while the greater the frequency the higher the pitch.

The difference between the two wave types is always stated with reference to the direction of energy transfer, never to the page or to the ground. In a longitudinal wave the oscillations or vibrations are parallel to the direction of energy transfer; in a transverse wave they are perpendicular to it. That single contrast explains the rest. Transverse waves are drawn with crests and troughs; longitudinal waves are drawn as compressions and rarefactions along the line of travel. Sound in air is longitudinal, while water ripples and every electromagnetic wave are transverse. Both types transfer energy from place to place without transferring any matter with it. Where two marks are offered, give both the parallel and the perpendicular statements; a pair of examples alone will not reach full marks.

The evidence is that the medium stays where it is while the wave travels on. On water, a floating object bobs up and down as ripples pass but stays in the same place, and pond weed rocks without being carried to the bank; if the water moved with the wave, the far end of the pond would fill up. For sound, a vibrating drum skin does not push all the air away in front of it, so no vacuum forms behind the wave, and a loudspeaker playing for an hour does not empty the room of air or create a wind. In both cases the particles oscillate about a fixed position, and only energy moves forward. A second sound example is dust or a candle flame near a loudspeaker: it vibrates back and forth about a fixed point rather than being blown across the room, showing the air oscillates but does not travel.