waves

transverse waves - a wave in which the medium vibrates at right angles to the direction of its propagation.

longitudinal waves - a wave vibrating in the direction of propagation.

Waves occur when a disturbance at the source of the wave causes particles to oscillate about a fixed central point.  There is a maximum displacement from the central point, which is called the equilibrium position.  When particles reach that maximum displacement, they start to move towards the central point. They pass through the central point as they move to the maximum displacement on the other side.

We can show this on a water wave.  The particles of water oscillate up and down from the equilibrium position.  The wave is travelling from left to right.  P is going down, Q is at the maximum displacement, and R is going up.

The wave is called a progressive wave because it is moving in a particular direction.  It is transferring energy from the point of disturbance, but the particles are not travelling with the wave, merely going up and down.

Waves can be considered to travel either as plane wave-fronts, from a plane source or as circular wave-fronts from a point source:

In 3 dimensions, the waves would propagate spherically from a point source.

Terms Used with Waves

Displacement of a particle is the distance at any given moment from the central or equilibrium position, i.e. the undisturbed position.  It is given the Physics Code s or x, and the SI unit is metre (m).  The displacement decreases the further the wave progresses from its source.

• Intensity of waves at a point is the power per unit area at that point.  The energy of a wave increases as the square of its amplitude.  However the energy decreases as the square of the distance from the source, which is known as the inverse square law.  The physics code for intensity is I and the units are watts per square metre (W m^-2).

• Amplitude of a wave, code A or r, units metres (m), is the maximum displacement of a particle from its equilibrium position.  In other words it is the height of the wave from the average level.  It is NOT the height from crest to trough.  (NB: Be careful of the code.  Here amplitude is given the code A, but in many texts you will see a.  This could be confused with acceleration.)

• Wavelength is defined as the distance between any two points on adjacent cycles that are in phase, in other words the distance between adjacent peaks or troughs.  The code for wavelength is l (lambda, a Greek letter ‘l’).  The units for wavelength are metre (m).

• Frequency, code f, has the unit hertz (Hz), and is the number of waves passing a given point every second.

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