explain that waves transfer energy without transferring matter
distinguish transverse and longitudinal waves, with examples
define amplitude, wavelength, frequency and period, and read them from graphs
use wave speed = frequency × wavelength and period = 1 ÷ frequency
relate pitch and loudness to frequency and amplitude, and use echoes to find distances
describe reflection, refraction and diffraction
Before you start
You will use these skills. If any feel shaky, review them first.
speed = distance ÷ time (see Speed)
rearranging equations, and prefixes such as kilo (k) and mega (M)
reading values from graphs
Key vocabulary
Amplitude
The maximum displacement from the rest position.
Wavelength (λ)
The distance from one point on a wave to the same point on the next wave, such as crest to crest.
Frequency (f)
The number of waves passing a point each second, in hertz (Hz).
Period (T)
The time for one complete wave. T = 1 ÷ f.
Transverse / longitudinal
Vibrations at right angles to / parallel to the direction of energy transfer.
Refraction / diffraction
A change of direction when waves change speed / spreading out through a gap or round an obstacle.
Understanding waves
What is it?
A wave is a travelling disturbance that transfers energy. In a transverse wave (light, ripples, waves on a rope) the vibrations are at right angles to the energy transfer. In a longitudinal wave (sound) they are along it, forming compressions and rarefactions.
wave speed = frequency × wavelength
Why does it happen?
Each vibrating particle pushes or pulls on its neighbours, so the disturbance is passed along while each particle only moves back and forth around its own position. In each second, f waves each λ long leave the source, so the wave front moves f × λ metres per second.
How do we know?
Ripple tanks, slinkies and oscilloscopes make wave motion visible. Seismologists noticed that one type of earthquake wave never reaches the far side of Earth; since those waves cannot pass through liquid, this was key evidence that Earth's outer core is liquid.
Why does it matter?
Waves carry almost all our communication, from speech to radio and mobile data. Echoes let doctors see inside the body with ultrasound and ships map the sea floor, and understanding ocean waves helps warn coasts of tsunamis.
What does it connect to?
Wave properties lead to the electromagnetic spectrum, light and lenses, and sound and hearing in biology. In mathematics they connect to sine graphs, rearranging formulae and standard form.
Reading wave graphs
Two kinds of graph look alike but show different things. Always check the horizontal axis.
Displacement–distance (Figure 1): a snapshot of the whole wave. Read the wavelength (crest to crest, arrow R) and the amplitude (rest position to crest, arrow P). Crest to trough (Q) is twice the amplitude.
Displacement–time (Figure 2): how one point moves. Read the period, then frequency = 1 ÷ period.
Figure 2. One point on the water: four complete vibrations in 2 s.
When a wave passes into a new medium, its speed changes but its frequency, set by the source, does not; so its wavelength changes too.
Waves in the real world
Musicians tune instruments by changing string tension, which changes the frequency. Sonar on ships times echoes to map the sea floor. Engineers design concert halls so that reflected sound arrives quickly enough to add richness without creating distracting echoes.
Worked examples
Example 1: using the wave equation
A mobile phone signal has a frequency of 900 MHz and travels at 300 000 000 m/s. Calculate its wavelength.
Figure 1 and Figure 2 describe the same kind of wave. If a wave has the wavelength in Figure 1 and the frequency in Figure 2, what is its speed?
From Figure 1: wavelength = 20 cm = 0.20 m.
From Figure 2: period = 0.5 s, so frequency = 1 ÷ 0.5 = 2 Hz.
speed = 2 Hz × 0.20 m = 0.40 m/s.
In the eAssessment
Waves questions often give you a graph or a table of measurements and ask you to extract values, calculate, then explain or evaluate. Expect:
Graph reading: decide first whether the graph is against distance (wavelength) or time (period).
Calculate: rearrange v = f λ and T = 1 ÷ f, convert prefixes (k, M) and units (cm to m), and give units in your answer.
Explain: use “the frequency is set by the source” when waves change medium, and “sound needs particles” for vacuum questions.
Evaluate: judge methods such as echo timing, focusing on reaction time and how the method reduces it.
Common ways to lose marks: measuring amplitude from crest to trough; reading a wavelength from a displacement–time graph; forgetting that an echo travels there and back; and claiming higher-frequency waves travel faster.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: frequency and wavelength in a ripple tank
Partially guided investigation · about 50 minutes · groups of three
Research question
How does the frequency of the vibrating bar in a ripple tank (5 to 25 Hz) affect the wavelength of the ripples, at a constant water depth?
Scientific background
The speed of water waves depends on the depth of the water. If the depth stays the same, the speed should stay the same, whatever the frequency.
Hypothesis
Write your own hypothesis, using the wave equation to justify it.
Variables
State the independent and dependent variables, and identify at least three control variables, explaining how and why you will control each.
Apparatus
Ripple tank with a lamp above and white paper below, vibrating bar with adjustable frequency, ruler on the paper, camera or phone, stroboscope (optional).
Method
Fill the tank to an even depth of about 1 cm, checking with a ruler at each corner.
Set the bar to 5 Hz and photograph the shadows of the ripples on the paper, with a ruler in view.
From the photo, measure the length of 5 wavelengths and divide by 5.
Repeat at 10, 15, 20 and 25 Hz, then repeat the whole set.
Calculate frequency × wavelength for each result.
Safety. Keep water well away from the mains lamp and power supply, and wipe up any spills immediately. Do not look directly into the lamp.
Results from one group are used in the Criterion C questions below. Then evaluate: why measure five wavelengths rather than one?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
