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# Physics > Waves

Waves are a disturbance which moves through space. Mechanical waves such as waves which propagate through water need a medium (the water) for them to exist. Electromagnetic radiation waves such as light and radio waves do not need a medium and can travel through a vacuum.

Waves transfer energy from one point to another without permanently displacing particles within the medium. Think of mechanical waves as a Mexican wave at a football ground. Each person just stands up and down (like particles of the medium) but the wave travels around the stadium. The particles of the medium simply oscillate around a fixed point.

School science usually deals with two types of waves; Transverse and Longitudinal.

Transverse waves (see fig 1 below) happen when a guitar string is plucked or when water is disturbed and include electromagnetic waves. In these waves, the vibration is perpendicular to the direction of the wave. (Think Mexican wave- the vibrations are up and down and the wave appears to move along)

Fig 1:
Transverse wave

Longitudinal waves (see fig 2 below) act differently in that the vibrations are parallel to the wave. Sound is an example of a longitudinal wave. The speaker cone moves in and out and with it pressure waves form and travel away from the cone. Transverse and Longitudinal wave diagrams are below:

Fig 2:
Longitudinal wave

All waves can experience reflection, refraction, interference and diffraction. These interactions are explained below. All these can be clearly seen using a ripple tank.
• REFLECTION occurs when waves hit a reflective, non absorbing surface and bounce off in a different direction.
• REFRACTION explains the change in direction of waves that enter a new medium. For example when waves travel from glass to air, their direction may change.
• INTEFERENCE happens when two waves come into contact with each other resulting in a new wave pattern.
• DIFFRACTION is the bending and spreading of waves which emerge from an aperture. For example waves at sea which enter a narrow harbour bend as they emerge from the gap.
In the picture below we can see the Amplitude and Wavelength of a wave. It is important to be able to know the difference between these two if you are to understand how to calculate the speed of a wave or to fully understand the electromagnetic spectrum.

Fig 3:
Wavelength and amplitude

The Wavelength of a wave is usually denoted with the Lambda symbol (λ) whereas Amplitude is usually represented by a capital A.

Amplitude is a measurement of the magnitude of oscillation (how far the football fan stands up to perform the Mexican wave)

Wavelength is the measurement of the distance between two crests or two troughs. It generally has the unit Metres.