P12 Wave properties

P12.1 The nature of waves

Waves transfer energy or information without transferring matter.

Mechanical waves are vibrations travelling through a medium and include sound waves and seismic waves. Electromagnetic waves can travel through a vacuum at 300 000 km/s, don't require a medium, and include radio and light waves.

When waves travel through a substance, the substance itself doesn't travel.

Transverse waves have oscillations perpendicular to the direction of energy transfer. Longitudinal waves have areas of compression and rarefaction and have oscillations parallel to the direction of energy transfer.

Sound waves are longitudinal whilst electromagnetic waves are transverse.

P12.2 Properties of waves

A wave's amplitude is the maximum displacement of one point on the wave from its undisturbed position. The bigger the amplitude, the more energy the waves carry. 

The wavelength is the distance from one wave crest to another.

Frequency is the number of waves passing a fixed point every second and is measured in hertz, Hz. The period is the time taken for 1Hz (each wave passing a fixed point).

period, T (seconds,s) = 1/frequency,f (hertz, Hz)

A ripple tank studies water waves in controlled conditions. By moving the edge of a ruler up and down on the water, you form a straight wave (plane wave, which all move at the same speed and are the same distance apart).

wave speed, v (metres per second, m/s) = frequency, f (hertz, Hz) x wavelength, λ (metres, m)

To measure the speed of sound in air, bang two cymbals from a far distance, and use a stopwatch to measure the time interval between seeing the impact and hearing the sound. Repeat several times to get an average and use the equation speed = distance/time to calculate the speed.

P12.3 Reflection and refraction

Reflection and refraction can be investigated using a ripple tank.

Plane waves directed at the metal barrier (incident waves) are reflected by the barrier, unless the incident wavefront isn't parallel to the barrier before or after reflection, in which case the reflected wavefront moves away from the barrier at the same angle to the barrier as the incident wavefront.

Refraction is the change of direction of a wave when it crosses a boundary between two mediums. In a ripple tank, plane waves directed at a boundary between deep and shallow water at a non-zero angle are refracted when they cross the boundary.

Wavefronts move by wavelets moving forward together to recreate the wavefront.

When plane waves cross a boundary at a non-zero angle, each wavefront experiences a change in speed and direction. At a slower speed the refracted wavefronts are closer together and at a smaller angle than the incident wavefronts.

Refracted and incident wavefronts have the same frequency but different wavelengths.

When a wave is directed at a substance, whether it is reflected depends on the substance (ie surface) and wavelength. They could also be absorbed, heating the substance due to gaining energy. This decreases the amplitude of the waves because they carry less energy. Waves not…