OCR A Level Physics A: Wave motion, polarisation and wave effects (H556) – Revision Notes

These notes condense section 4.4.1 Wave motion, statements (a) to (g), of the OCR A Level Physics A (H556) specification, Version 3.0 (March 2026). For full explanations and longer worked examples, use the study guide. Test yourself afterwards with the practice questions. The course hub, the printable checklist and the free 10-minute diagnostics show where to focus.

Definitions

Term	Meaning
Progressive wave	Transfers energy from place to place; the medium’s particles only oscillate about fixed positions
Transverse	Oscillations perpendicular to the way energy moves
Longitudinal	Oscillations along the way energy moves; compressions and rarefactions
Displacement	Distance from the equilibrium position, with direction
Amplitude	Maximum displacement
Wavelength λ	Separation of neighbouring points oscillating in step
Period T	Duration of one full oscillation
Frequency f	Number of cycles each second (Hz)
Wave speed v	How far the wave pattern advances each second
Phase difference	How far apart in their cycles two points are
Intensity	Power passing through unit area facing the wave

Equations

Equation	Use
f = 1/T	Frequency and period
v = fλ	Wave speed
phase difference = 360x/λ degrees = 2πx/λ radians	Points a distance x apart on a wave
I = P/A	Intensity, in W m⁻²
I = P/(4πr²)	Isotropic point source, distance r
I ∝ (amplitude)²	Compare waves, or one wave at two places

Quote a phase difference as a part of a cycle, or as an angle in degrees or radians. In phase means 0 or a whole number of cycles. Antiphase means half a cycle (180° or π rad).

Graphs

• Displacement against distance shows the wave at one instant. Read λ and amplitude.
• Displacement against time shows one point as time passes. Read T and amplitude.
• For a longitudinal wave, plot displacement of particles along the direction of travel (right positive). A compression is at a zero crossing where the graph slopes downwards. A rarefaction is at a zero crossing sloping upwards.
• The centre of a compression has particles squeezed together, so a graph peak (maximum displacement) is not a compression.

Oscilloscope method (find frequency)

1. Connect the signal and set the time base so that several cycles show.
2. Adjust Y-gain and trigger for a steady, large trace.
3. Count the horizontal divisions for n cycles and divide by n.
4. T = divisions per cycle × time base. Then f = 1/T.
5. Amplitude is half the peak-to-peak height in divisions, times the Y-gain.

Worked reminder: 2.5 divisions per cycle at 4.0 ms per division gives T = 10 ms and f = 100 Hz.

Wave effects

Effect	What happens	What stays the same
Reflection	Wave bounces off a boundary; angle of incidence equals angle of reflection	f, v, λ
Refraction	Direction changes when speed changes at a boundary	f (v and λ change together)
Diffraction	Wave spreads through a gap or round an obstacle	f, v, λ
Polarisation	Oscillations restricted to one plane (transverse waves only)	f, v, λ

Diffraction rule. The effect is significant when the wavelength is comparable to the gap width. Gap much larger than λ: little spreading. Gap about λ: wide spreading. Longer wavelengths therefore bend round obstacles more easily than shorter ones.

Ripple tank method

1. A dipper vibrating at a known frequency makes waves in shallow water. A lamp above projects the pattern onto a screen below.
2. Use a stroboscope matched to the dipper frequency to freeze the pattern.
3. Measure across several wavelengths with a ruler and divide, then correct for magnification.
4. Reflection: place a straight barrier in the path.
5. Diffraction: leave a gap between two barriers and narrow it towards λ.
6. Refraction: place a flat plate on the bottom to make a shallow region. The wavelength shortens and the wavefronts bend.
7. v = fλ gives the speed.

Polarisation methods

Microwaves. Transmitter and receiver aligned, then rotate the receiver. The reading falls close to zero after a quarter turn and recovers. A metal grille between them transmits the signal when the rods are perpendicular to the oscillations and blocks it when they are parallel.

Light. Two Polaroid filters. Rotate one: the transmitted light dims to a minimum and recovers, and over a full turn this happens twice. Crossed filters (90°) pass almost no light. Reflected glare is partly polarised, so Polaroid sunglasses reduce it.

What polarisation shows. Only transverse waves can be polarised. If a wave can be polarised, it is transverse. Sound cannot be polarised.

Intensity reminders

• Doubling the distance from an isotropic source cuts intensity to one quarter. Tripling cuts it to one ninth.
• Doubling the amplitude gives four times the intensity. Halving amplitude gives one quarter.
• Power collected = intensity × area of the receiver.
• Worked reminder: 35 W from an isotropic source at 4.0 m gives I = 35/(4π × 4.0²) = 0.174 W m⁻².

Must-know distinctions

• Frequency and speed. The source fixes the frequency. The medium fixes the speed. The wavelength follows from v = fλ.
• Period and wavelength. T is a time and belongs on a time axis. λ is a length and belongs on a distance axis.
• Wave speed and particle speed. The pattern moves at v. Each particle oscillates about a fixed point and its velocity keeps changing.
• Amplitude and intensity. Amplitude is a displacement. Intensity is power per unit area, and it depends on the square of the amplitude.
• Polarised and unpolarised. An unpolarised wave has oscillations in many planes. A polarising filter lets one plane through, so the transmitted wave has lower intensity than the incident wave.
• Diffraction and refraction. Diffraction is spreading at a gap or edge with no change of wavelength. Refraction is a change of direction because the speed changed.

Worked reminder: phase and graphs

A displacement-distance graph shows a wave with crests at x = 0.10 m and x = 0.50 m. The wavelength is 0.40 m. A point at x = 0.30 m is half a wavelength from a crest, so it is in antiphase with the crest, a phase difference of 180° (π rad). A point at x = 0.60 m is 0.10 m from the next crest position, so it lags or leads by 0.10/0.40 = 0.25 of a cycle, which is 90°.

Quick self-test

1. A wave has a period of 8.0 ms. Find its frequency.
2. Sound of frequency 1500 Hz has wavelength 0.23 m in a gas. Find the speed of the wave.
3. Two points on a wave are 0.15 m apart and the wavelength is 0.60 m. State the phase difference in degrees.
4. Say what makes a wave longitudinal, and name one example.
5. A water wave enters a shallower region. State which of f, v and λ change.
6. Why can you hear a person speaking round a corner but not see them?
7. State why sound waves cannot be polarised.
8. Two Polaroid filters are crossed. Describe what a person sees looking through both at a lamp.
9. An isotropic source of power 35 W is used. Calculate the intensity at 4.0 m.
10. A wave’s amplitude falls to half its original value. By what factor does its intensity change?
11. A trace on an oscilloscope has one cycle across 5.0 divisions. The time base is 2.0 ms per division. Find the frequency.
12. Give the formula for the intensity of a wave in terms of power and area, with its unit.

Answers

1. f = 1/T = 1/(8.0 × 10⁻³) = 125 Hz.
2. v = fλ = 1500 × 0.23 = 345 m s⁻¹.
3. 0.15/0.60 = 0.25 of a cycle, so 0.25 × 360 = 90°.
4. The oscillations lie along the way the energy moves, for example sound.
5. v and λ decrease. f does not change.
6. Sound has a wavelength comparable to a doorway, so it diffracts round the edge. Light has a wavelength far smaller than the gap, so it hardly diffracts.
7. Sound is longitudinal, so its oscillations are along the direction of travel and there is no sideways plane to select.
8. Almost no light passes through both, so the lamp looks dark.
9. I = P/(4πr²) = 35/(4π × 16.0) = 0.174 W m⁻².
10. Intensity ∝ amplitude², so it falls by a factor of 4 (to 0.25 of the original).
11. T = 5.0 × 2.0 × 10⁻³ = 1.0 × 10⁻² s, so f = 100 Hz.
12. I = P/A, in watts per square metre (W m⁻²).

Where marks are usually lost

• Writing “light is a transverse wave so it can be polarised” without saying what polarisation does to the oscillations.
• Reading T from a distance graph or λ from a time graph.
• Not multiplying divisions by the time base, or forgetting that a trace shows more than one cycle.
• Saying frequency changes on refraction, or that wavelength stays constant.
• Describing diffraction as needing a “small” gap with no comparison to the wavelength.
• Saying intensity doubles when amplitude doubles.
• Leaving out units, or giving a phase difference with no unit (degrees or radians).
• In ripple tank method answers, not saying how the pattern was frozen or how the magnification was dealt with.
• Calling a graph peak of a longitudinal wave a compression.

Official syllabus

OCR A Level in Physics A (H556) specification, Version 3.0 (March 2026), first assessment 2017, published by OCR: section 4.4.1 Wave motion, statements (a) to (g).