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Types of waves

Waves are one of the ways in which energy may be transferred between stores. All waves transfer energy but they do not transfer matter.

Waves can be described as oscillations or vibrations about a rest position. For example:

• sound waves cause air particles to vibrate back and forth
• ripples cause water particles to vibrate up and down

The direction of these oscillations is the difference between longitudinal or transverse waves. In longitudinal waves, the vibrations are parallel to the direction of wave travel. Examples of longitudinal waves include sound waves and seismic P-waves (primary waves). In transverse waves, the vibrations are at right angles to the direction of wave travel. Examples of transverse waves include electromagnetic radiation, water waves and seismic S-waves (secondary waves).

Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a medium to travel through. Electromagnetic waves cause oscillations in electrical and magnetic fields.

Waves are described using the following terms:

• rest position - the undisturbed position of particles or fields when they are not vibrating
• displacement - the distance that a certain point in the medium has moved from its rest position
• peak (crest) - the highest point above the rest position
• trough - the lowest point below the rest position
• amplitude - the maximum displacement of a point of a wave from its rest position
• wavelength - distance covered by a full cycle of the wave, usually measured from peak to peak, or trough to trough
• time period - the time taken for a full cycle of the wave, usually measured from peak to peak, or trough to trough
• frequency - the number of waves passing a point each second
• wavefront - the imaginary plane of waves cutting through them in the same position. This could be where all the peaks (crests) are, all the troughs are or any place in between.

Waves cause a disturbance of the medium through which they travel. This allows them to carry energy. The quantity of energy carried relates to the amplitude of the wave.

In transverse waves, the particles of the medium vibrate at right angles to the direction that the energy travels. This is where the name transverse comes from - it means across. All of the waves in the electromagnetic spectrum are transverse waves, as are water waves.

The following types of waves are transverse waves:

• ocean waves

• light waves

• microwaves

• radio waves

• ultraviolet radiation

Transverse waves can also be made by shaking a rope up and down or from side to side and look like this:

In the diagram above the rope moves up and down but the energy is transferred along the rope from left to right, at 90° to the oscillation.

In a transverse wave, the oscillation (vibration) is at right angles to the direction of energy transfer.

• The particles move up and down as the wave moves from left to right.

• However, none of the particles are transported along a transverse wave.

Sound waves, pressure waves and primary waves (a type of seismic wave produced by earthquakes), are longitudinal waves. Longitudinal waves can also be made by pushing a spring forwards and backwards. They look like this:

Producing longitudinal waves using a spring

In a longitudinal wave, the vibration is parallel to the direction of energy transfer.

In the diagram above, each individual coil of the slinky moves backwards and forwards either side of its rest position. However, none of the coils are moved along the length of the slinky. They just move backwards and forwards parallel to the direction in which the energy flows. The energy is transferred along the slinky, parallel to the oscillation.

A ripple tank can be used to investigate the frequency, wavelength and the speed of water waves.

A ripple tank is a transparent shallow tray of water with a light shining down through it onto a white card below.

The light allows you to see the motion of the ripples created on the water's surface more easily.

Ripples can be made by hand but to generate regular ripples it is better to use a motor.

Waves - including water waves, sound and light - can be reflected at the boundary between two different materials.

When drawing wave diagrams, it is easier to draw wave fronts rather than crests and troughs.

Each wavefront is drawn at right angles to the wave direction.

For wave diagrams, the wave direction and wave fronts are normally drawn.

Refraction is the change in direction of a wave as it travels from one medium to another.

For instance, when light travels from air to glass or water waves travel from deep to shallow water.

Key point

FAST is a useful way of remembering the speed and direction changes of waves during refraction:

• If waves get Faster, they bend Away from the normal.

• If waves get Slower, they bend Towards the normal.

When waves meet a gap or an edge in a barrier, they continue through the gap or past the edge of the barrier. However, what happens on the far side of the gap or barrier is not so straightforward.

The waves always 'spread' to some extent into the area beyond the gap.

This is diffraction - the spreading out of waves when they go through a gap, or past the edge of a barrier.

Test your knowledge with this quiz on the general properties of waves.

Are you a physics teacher looking for more resources? Share these short videos with your students:

• Measure the frequency, wavelength and speed of waves in a ripple tank

• The GB surfing team demonstrate different types of waves

• Refraction of light