C.2.2—Wave quantities and wave equation

Define the quantities

Wavelength $\lambda$ is the shortest distance between points in phase. Frequency $f$ is cycles per second, period $T$ is the time for one cycle, and amplitude is maximum displacement from equilibrium. Wave speed $v$ is the speed at which the disturbance and energy propagate.

Connect time and space

For a travelling wave, $v=f\lambda=\lambda/T$ . Use a spatial wavelength measured in metres and a temporal frequency measured in hertz; the result is in metres per second.

Read the same ideas in both wave types

For transverse waves, wavelength can be measured crest-to-crest or trough-to-trough. For longitudinal waves, measure compression-to-compression or rarefaction-to-rarefaction. In both cases the points are in phase.

Common trap

Do not use the distance from a crest to the next trough as one wavelength; that is half a wavelength. Do not confuse the speed of the medium particles with the propagation speed $v$ .

How it is assessed

Questions ask you to state wavelength and period from a sound-wave representation or calculate a wave quantity. The evidence rewards correct same-phase spacing and consistent SI units.

Command terms

State

What earns marks

Identify λ from same-phase points, obtain T or f from the time data, and use v = fλ = λ/T. State units for wavelength and period, and distinguish wave propagation speed from the local oscillation speed of the medium.

Watch for

Using crest-to-trough spacing as the wavelength or reporting frequency when the question asks for period.

Representative question

[Maximum number: 2]

State the wavelength and the period of the sound wave.

Model the transfer

A travelling wave propagates a disturbance and transfers energy without a resultant transport of the medium. Use the local particle/field motion to describe oscillation, and the wave direction to describe propagation.

Connect the quantities

Describe waves with wavelength $\lambda$ , frequency $f$ , period $T$ , amplitude and speed $v$ , linked by $v=f\lambda=\lambda/T$ . Measure wavelength between adjacent in-phase points.

Classify the wave

Transverse oscillations are perpendicular to propagation; longitudinal oscillations are parallel. Mechanical waves require a medium, while electromagnetic waves are transverse field oscillations and travel at $c$ in vacuum.

Final check

At a boundary, identify which quantity is fixed by the source and which properties change in the new medium. Keep units consistent and distinguish propagation speed from the local oscillation speed.

C.2.2—Wave quantities and wave equation

Describe Wave Quantities

C.2.2 Exam Analysis

Assessment in practice

Retrieve the C.2 Wave Model

Question 1