Wave properties and calculations

Waves may be either transverse or longitudinal.

The ripples on a water surface are an example of a transverse wave. Longitudinal waves show areas of compression and rarefaction. Sound waves travelling through air are longitudinal.

You should be able to describe the difference between longitudinal and transverse waves. The difference is the direction the particles move relative to the direction the wave travels: in a transverse wave they move at right angles to it, and in a longitudinal wave they move along the same line as it.

You should be able to describe evidence that, for both ripples on a water surface and sound waves in air, it is the wave and not the water or air itself that travels.

For water: a floating object such as a cork or a duck bobs up and down as ripples pass under it, but it does not travel along with them. If the water itself were moving forwards, the cork would be carried to the far side.

For sound: sound travels across a room, but there is no wind — the air is not moving from the speaker to your ear. If it were, a candle in front of a loudspeaker would be blown out, and the room would gradually empty of air.

What a wave transfers is energy, and information, from one place to another — not matter.

You should be able to describe wave motion in terms of their amplitude, wavelength, frequency and period.

• The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position. • The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave. • The frequency of a wave is the number of waves passing a point each second.

The wave speed is the speed at which the energy is transferred, or the wave moves, through the medium.

period = 1 / frequency T = 1 / f

period, T, in seconds, s; frequency, f, in hertz, Hz. This equation is given on the Physics equation sheet.

All waves obey the wave equation:

wave speed = frequency × wavelength v = f λ

wave speed, v, in metres per second, m/s; frequency, f, in hertz, Hz; wavelength, λ, in metres, m.

You should be able to recall and apply the wave equation — note the difference: v = f λ must be recalled, while T = 1 / f is given.

You should also be able to identify amplitude and wavelength from given diagrams. Amplitude is measured from the undisturbed middle line to a peak, not from a peak to a trough.

You should be able to describe a method to measure the speed of sound waves in air, and a method to measure the speed of ripples on a water surface.

Sound in air: two people stand a measured distance apart — a few hundred metres, so the time is long enough to measure. One bangs two blocks together while the other starts a stopwatch on seeing the bang and stops it on hearing it. Repeat and take a mean, then use speed = distance ÷ time. The result should be around 330 m/s.

Ripples on water: use a ripple tank with a motorised dipper. Measure the wavelength by freezing the pattern with a strobe light and measuring across ten wavelengths, then dividing by ten. Read the frequency from the dipper's setting, or count the waves passing a point in a measured time. Then use v = f λ.

Physics only students should be able to show how changes in velocity, frequency and wavelength, in transmission of sound waves from one medium to another, are inter-related.

The rule to hold on to: when a wave crosses into a different medium, the frequency does not change — it is set by the source. The speed does change. So from v = f λ, if the speed increases the wavelength must increase in proportion, and if the speed decreases the wavelength decreases.