Students should be able to describe the difference between longitudinal…

The idea

Waves transfer energy from one place to another without transferring matter. The key difference is the direction in which the particles of the medium vibrate compared with the direction in which the wave travels. In a transverse wave, the vibrations are at right angles (perpendicular) to the direction of energy transfer; examples include light, water surface ripples and waves on a rope shaken up and down. In a longitudinal wave, the vibrations are along the same line as the direction of energy transfer, producing regions of compression and rarefaction; sound in air is the standard example. A useful check is to trace the wave direction and then ask whether the oscillation is across it or back and forth along it. Transverse waves can be polarised, whereas longitudinal waves cannot, which is a further distinguishing feature.

What earns the marks

• States that in a transverse wave the oscillations or vibrations are perpendicular (at right angles) to the direction of energy transfer or wave travel.
• States that in a longitudinal wave the oscillations or vibrations are parallel to the direction of energy transfer or wave travel.
• Identifies compressions and rarefactions as the regions of high and low pressure or density in a longitudinal wave.
• Gives a correct example for each type, such as light or water ripples for transverse and sound in air for longitudinal.
• Recognises that both wave types transfer energy without transferring matter overall.
• Notes that transverse waves can be polarised while longitudinal waves cannot.

Common mistakes and how to put them right

• Confusing the two types by saying transverse waves vibrate along the direction of travel.

  Put it right by linking transverse to perpendicular and longitudinal to parallel.

• Thinking that the medium itself travels with the wave.

  Put it right by stressing that particles oscillate about fixed positions while energy is transferred.

• Describing compressions as places where particles are stationary.

  Put it right by explaining that particles in a compression are closer together and still vibrating.

• Assuming all waves are transverse.

  Put it right by recalling that sound and some seismic waves are longitudinal.

Exam tips

• Define the reference direction first: state the direction of energy transfer, then describe the vibration direction relative to it.
• Use the words perpendicular and parallel explicitly, since vague phrases such as 'side to side' may not gain credit.
• Support each definition with one named example to show understanding rather than repeating the definition.
• If a diagram is given, annotate the vibration direction and the wave travel direction with arrows before writing your answer.