Longitudinal waves

Longitudinal waves are defined as 

“Waves which vibrate parallel to the direction of energy transfer”

Longitudinal waves: 

  • vibrate in the same direction as the direction of travel

  • transfer energy, but not matter 

  • can move in solids, liquids and gases

  • cannot move in a vacuum as there are no particles

The key features of a longitudinal wave are where the points are:

  • close together, called compressions

  • spaced apart, called rarefactions

                                                                          Example of a longitudinal wave

Longitudinal waves can be demonstrated in a slinky spring when it is moved quickly backwards and forward

Examples of longitudinal waves are:

  • sound waves

  • P-waves (a type of seismic wave)

  • pressure waves caused by repeated movements in a liquid or gas

Representing longitudinal waves

Longitudinal waves are usually drawn as several lines to show that the wave is moving parallel to the direction of energy transfer

  • Drawing the lines close together represents the compression

  • Drawing the lines further apart represents the rarefactions

Longitudinal wave rarefactions and compressions

Longitudinal waves are represented as sets of lines with rarefactions and compressions

Transverse v Longitudinal Waves

Wave vibrations can be demonstrated on ropes (transverse) and springs (longitudinal)

                                   A comparison of longitudinal and transverse waves

Wave motion can be demonstrated through vibrations in ropes or springs

 Property

Transverse waves

Longitudinal waves

 Structure

Peaks and troughs

Compressions and rarefactions 

 Vibration

Right angles to the directions of energy transfer 

 Parallel to the direction of energy transfer

 Vacuum

Can travel in a vacuum (electromagnetic waves only) 

Cannot travel in a vacuum 

 Material 

Can move in solids on the surface of liquids

 Can move in solids, liquids and gases

 Pressure

 Pressure is constant

Pressure is not constant 

Speed of wave 

 Depends on the material it is travelling through (fastest in a vacuum)

 Depends on the material it is travelling through (fastest in a solid)

Wavefronts

Both transverse and longitudinal waves can be represented as wavefronts

  • This is where the waves are viewed from above

For a transverse wave: 

  • One line represents either a peak or trough

For a longitudinal wave:

  • One line represents either a compression or  rarefaction

The arrow shows the directions the wave is moving and is sometimes called a ray

The space between the lines represents the wavelength

  • When the lines are close together, this is a wave with a short wavelength

  • When the lines are far apart, this is a wave with a long wavelength

                     Wavefronts as viewed from above

Diagram showing a wave moving to the right drawn as a series of wavefronts

Examiner tips and tricks

Exam questions may ask you to describe waves, and this is most easily done by drawing a diagram of the wave and then describing the parts of the wave - a good, clearly labelled diagram can earn you full marks!

Make sure you know the difference between the wave front diagram and the longitudinal wave diagram; do not confuse the two!