Transverse and longitudinal waves, amplitude, wavelength, frequency, period and speed, the wave equation, phase and phase difference, and polarisation of transverse waves.

What this dot point is asking

Edexcel wants you to distinguish transverse and longitudinal waves, define amplitude, wavelength, frequency, period and speed, apply the wave equation $v = f\lambda$, work with phase and phase difference, and explain polarisation of transverse waves.

The answer

Transverse and longitudinal waves

Light and other electromagnetic waves, and waves on a string, are transverse. Sound and seismic P-waves are longitudinal. In both, individual particles oscillate about a fixed point while the energy travels onward.

Describing a wave

The amplitude of a wave determines its intensity: intensity is proportional to the square of the amplitude. Frequency is set by the source and does not change when the wave moves into a new medium, even though the speed and wavelength do.

The wave equation

This single equation links the three core wave quantities and is used constantly. It follows directly from the definition of speed: in one period the wave moves forward one wavelength.

Phase and phase difference

Phase describes the stage a point has reached in its cycle. Two points exactly one wavelength apart are in phase (phase difference of $360$ degrees or $2\pi$ radians); half a wavelength apart they are in antiphase ($180$ degrees or $\pi$ radians). Phase difference can be found from path difference: a path difference of $\lambda$ corresponds to a phase difference of $2\pi$.

Polarisation

Passing unpolarised light through one filter polarises it; a second filter (analyser) at $90$ degrees to the first blocks it entirely. This is direct evidence that light is a transverse wave, since sound (longitudinal) cannot be polarised.

Examples in context

Polaroid sunglasses cut glare by blocking the horizontally polarised light reflected from roads and water. The wave equation underpins all of acoustics and optics, from tuning instruments to designing antennas. Seismologists distinguish longitudinal P-waves from transverse S-waves to locate earthquakes and probe the Earth's interior (S-waves cannot pass through the liquid outer core). Phase difference is central to interference, sonar, and the way stereo sound is recorded and reproduced.

Try this

Q1. State the wave equation. [1 mark]

• Cue. $v = f\lambda$, wave speed equals frequency times wavelength.

Q2. A radio wave has frequency $1.0 \times 10^{8}$ Hz and travels at $3.0 \times 10^{8}$ m per second. Find its wavelength. [2 marks]

• Cue. $\lambda = \frac{v}{f} = \frac{3.0 \times 10^{8}}{1.0 \times 10^{8}} = 3.0$ m.

Q3. Explain why sound cannot be polarised. [2 marks]

• Cue. Sound is longitudinal, oscillating along the direction of travel, so there is no perpendicular plane of oscillation to restrict.