Transverse & Longitudinal Waves
- In mechanical waves, particles oscillate about fixed points
- There are two types of wave: transverse and longitudinal
- The type of wave can be determined by the direction of the oscillations in relation to the direction the wave is travelling
Transverse Waves
- Transverse waves are defined as follows:
A wave in which the oscillations are perpendicular to the direction of motion and energy transfer
A transverse wave travelling from left to right
- This means that each particle in the wave vibrates up and down
- Transverse waves show areas of peaks and troughs
- Examples of transverse waves include:
- Electromagnetic waves e.g. radio, visible light, UV
- Vibrations on a guitar string
- Transverse waves transfer energy, even if there is no resultant displacement of the medium
- This means transverse waves do not need particles to propagate, so they can travel through a vacuum
- This is why we can still feel the UV radiation from the Sun, as it can travel through the vacuum of space
Longitudinal Waves
- Longitudinal waves are defined as follows:
A wave in which the oscillations are parallel to the direction of motion and energy transfer
A longitudinal wave travelling from left to right
- This means that each particle in the wave only vibrates left and right
- As a longitudinal wave propagates, areas of low and high pressure can be observed:
- A rarefaction is an area of low pressure, with the particles being further apart from each other
- A compression is an area of high pressure, with the particles being closer to each other
Rarefactions and compressions on a longitudinal wave
- Sound waves are an example of longitudinal waves
- Longitudinal waves need particles to propagate, so they cannot travel through a vacuum
- This is why you cannot hear anything in the vacuum of outer space
Worked example
The diagram below represents a transverse wave at time t = 0. The direction of motion of the wave is shown. Point P is a point on the wave. State in which direction point P will move immediately after the time shown.
Answer:
Step 1: Determine the possible directions that point P can travel in
- In transverse waves, the particles oscillate perpendicular to the direction of motion
- This transverse wave travels from right to left
- Oscillations will either be up or down
- Hence point P will either move up or down
Step 2: Determine the next direction of point P
- Since the wave is moving from right to left, a crest (i.e. a point of maximum displacement above the equilibrium position) will be approaching point P immediately after the time shown
- Point P will be moving upwards
Examiner Tip
Exam questions will focus on the description of the motion of particles of a medium when a wave passes through it, for both types. Make sure you remember the difference between them:
- Particles in a transverse wave move up and down
- Particles in a longitudinal wave move left and right
The particles do not travel 'along' the wave, they are just in one position, and can only move either vertically or horizontally. All the particles have the same motion, but are displaced slightly, creating the illusion that the whole wave is moving together.
