Transverse Waves
For transverse waves the displacement of the particles is perpendicular to the direction of wave travel. In other words the oscillations are perpendicular to the direction of wave propagation.
Examples of transverse waves include all electromagnetic waves, ripples on the surface of water and vibrations along a string.
IMPORTANT NOTE: The particles do not move along a transverse wave. They just move up and down about the rest position.
Let’s look at a particle in the medium this wave is travelling in. The diagram below shows this particle in red:
As the wave moves along the particle moves up and down, at right angles to the direction of travel:
For simplicity these diagrams show the particle at key points as the wave travels – the crest of the wave, rest position and trough. In reality the particle transitions smoothly up and down as the oscillations pass through the medium (i.e. it doesn’t just jump from the crest position to the rest position, and from the rest position to the trough).
As you can see the particle doesn’t move along with the wave. The wave is moving left to right in the diagrams and this transmits energy throughout the medium.
Experiment
To visualise this take a piece of string and fix one end to a table. Hold the other end in line with the fixed point.
The horizontal line is rest position. Now if you move the end you are holding up and down you will create a transverse wave along the the string. The wave disturbance will move horizontally towards the other end of the string, but each point along the string moves up and down perpendicular to the rest position.
Longitudinal Waves
For longitudinal waves the displacement of the particles is parallel to the direction of wave travel. In other words the oscillations are parallel to the direction of wave propagation.
Sound waves are an example of longitudinal waves.
Longitudinal waves produce regions of compression and rarefaction:
Compression – particles are close together and pressure is higher.
Rarefaction – particles are further apart and pressure is lower.
IMPORTANT NOTE: The particles do not move along a longitudinal wave. They just move backwards and forwards between compressions. This means that a particle at the start of a longitudinal wave moves back and forth near the start position and does not travel to the waves end position.
In the diagram the wave moves from left to right and this transmits energy throughout the medium.
The wavelength of a longitudinal wave is the distance between two consecutive points that are in phase with each other. The easiest way to measure this is to measure the distance between two consecutive compressions:
Experiment
To visualise a longitudinal wave get a slinky and ask someone to hold one end in a fixed position. Hold the other end of the slinky and you can create a longitudinal wave by pushing and pulling the slinky horizontally.
As the wave moves along the slinky you will see the distance between the coils decrease and increase. When the coils are closer together this is a compression and when they are further apart is a rarefaction. Each compression will move along to the end of the slinky, transferring the energy from start to finish.
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