Types of wave - CCEA

• To understand the difference between transverse and longitudinal waves.

• To identify the different parts of a wave.

• To be able to understand the different graphs used for waves.

Waves transfer energy from one place to another through vibrations(oscillations).

For example:

• Ripples cause water particles to vibrate up and down.

• Sound waves cause air particles to vibrate back and forth.

The direction of these oscillations is the difference between longitudinal and transverse waves.

A transverse wave is one in which the vibrations of the particles are perpendicular (at right angles) to the direction of travel of the wave.

How to demonstrate transverse waves

Transverse waves are often demonstrated by moving a rope rapidly up and down.

Energy is transferred at right angles to the hand movement.

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

Examples of transverse waves include:

• Ripples on the surface of water.

• Vibrations in a guitar string.

• A Mexican wave in a sports stadium.

• Electromagnetic waves – eg light waves, microwaves, radio waves, x-rays.

Mechanical waves cause oscillations of particles in a solid, liquid or gas and must have a medium to travel through – they cannot pass through a vacuum.

Electromagnetic waves cause oscillations in electrical and magnetic fields and can travel through a vacuum.

A longitudinal wave is one in which the vibrations of the particles are parallel to the direction of travel of the wave.

How to demonstrate longitudinal waves

Longitudinal waves show areas of compression and rarefaction:

• Compressions are regions of high pressure due to particles being close together.

• Rarefactions are regions of low pressure due to particles being spread further apart.

In the diagram, the compression moved from left to right and energy is transferred from left to right.

The movement of the coils of the slinky and the energy are parallel.

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

Instead, they move backwards and forwards as the wave is transmitted through the medium.

Examples of longitudinal waves include:

• Sound waves.

• Ultrasound waves.

One way of remembering examples of longitudinal waves is to emphasise the O in lOngitudinal and sOund and ultrasOund.

Key fact

All waves transfer energy from one place to another, but they do not transfer matter.

Waves are described using the following terms:

• Rest position (equilibrium 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 - 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 λ – distance from one point on a wave to the next identical point.

Wavelength is measured in m.

For a transverse wave it is usually measured from a peak to the next peak, or trough to the next trough.

For a longitudinal wave it is usually measured from a compression to the next compression, or rarefaction to the next rarefaction.

Time period T - the time taken for a full wave.

• Period is measured in s.
• A period of 2 s means that the time for one complete vibration is 2 s or that the time taken for one complete wave to pass a point is 2 s.

Frequency f - the number of waves passing a point each second.

• Frequency is measured in hertz Hz.
• A frequency of 50 Hz means there are 50 vibrations every second.

There are two types of graphs used to display information about a wave:

1. Displacement–distance graph:

As the x-axis is distance, the following can be measured from this graph:

• amplitude;
• wavelength.

2. Displacement-time graph:

As the x-axis is time, the following can be measured from this graph:

• amplitude;

• period.