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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 tranverse 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. Once the wave has passed, the medium ends up back in its rest position.

For example, after a wave reaches the beach, the sea ends up back where it started. Similarly, when a sound travels, there isn’t a wind that moves with it - the air ends up back where it started.

Mechanical waves can be longitudinal or transverse.

Electromagnetic waves cause oscillations in electrical and magnetic fields. All electromagnetic waves are transverse.

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.

Diagram of a wave