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Waves are one of the ways in which energy may be transferred between stores. 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. In transverse waves, the vibrations are at right angles to the direction of wave travel.

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.

It is important to remember that all waves transfer energy but they do not transfer matter. For example, if a ball is placed on the surface of a pond when ripples move across it, the ball will move up and down but not outwards with the wave. Similarly, when sound waves move through the air to a person’s ear, the air itself does not move - instead the sound is transferred through the vibrating molecules, which stay where they are and vibrate about a fixed point.

Parts of a wave

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 - 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.