OCR GCSE Physics A: Wave properties, transverse and longitudinal waves (J249) – Revision Notes

These notes condense statements P5.1a to P5.1e, P5.1j and P5.1k of OCR GCSE (9-1) Gateway Science Physics A (J249, Version 5.0, July 2026), topic P5.1 Wave behaviour. None of these statements is in bold in the specification, so Foundation and Higher Tier candidates both need all of them. For full explanations go to the study guide, and test yourself with the practice set. Course pages: the OCR GCSE Physics hub and the checklist. Not sure what to revise first? Use the free 10-minute diagnostics.

Key definitions (P5.1a, P5.1b)

Term	Symbol and unit	Meaning
Amplitude	metres (m)	Maximum displacement from the rest (middle) position
Wavelength	λ, metres (m)	Length of one complete wave, for example crest to crest
Frequency	f, hertz (Hz)	Number of waves passing a point per second; vibrations per second
Period	T, seconds (s)	Time for one complete wave to pass a point
Wave speed	v, metres per second (m/s)	Distance the wave moves each second
Medium		The material a wave travels through

Link between period and frequency: T = 1 ÷ f and f = 1 ÷ T.

Bigger amplitude means more energy carried. Amplitude does not change the wavelength or the frequency.

Formula and rearrangements (P5.1c, P5.1d)

Finding	Equation	Units
Wave speed	v = f × λ	m/s = Hz × m
Frequency	f = v ÷ λ	Hz
Wavelength	λ = v ÷ f	m
Frequency from period	f = 1 ÷ T	Hz

The specification’s mathematical outcomes list wave speed = frequency × wavelength to recall and apply.

Unit conversions to do before substituting:

• 1 kHz = 1000 Hz
• 1 cm = 0.01 m, 1 mm = 0.001 m
• 1 ms = 0.001 s

Method in steps

1. Write the given quantities with symbols and units.
2. Convert to Hz, m and s.
3. Rearrange first if needed, then substitute.
4. Write the answer with a unit.
5. Check the size: for the same speed, a higher frequency must mean a shorter wavelength.

Worked reminders

Frequency 120 Hz and wavelength 2.8 m give v = 120 × 2.8 = 336 m/s.

A note of 850 Hz in air (340 m/s, data supplied) has λ = 340 ÷ 850 = 0.40 m.

Ripples at 0.45 m/s with wavelength 1.5 cm = 0.015 m have f = 0.45 ÷ 0.015 = 30 Hz.

More reminders. An oscilloscope trace with 8 complete waves across 10 ms has T = 10 ÷ 8 = 1.25 ms, so f = 1 ÷ 0.00125 = 800 Hz. A pulse timed at 2.5 ms between microphones 0.85 m apart gives v = 0.85 ÷ 0.0025 = 340 m/s.

Reading graphs

Graph	Horizontal axis	Read off
Displacement against distance	Distance	Wavelength, amplitude
Displacement against time	Time	Period (then f = 1 ÷ T), amplitude

An oscilloscope trace is a displacement-time graph. Peak to peak along the axis is a period, not a wavelength. Amplitude is measured from the middle line, which is half the trough-to-crest height.

Transverse and longitudinal waves (P5.1e)

	Transverse	Longitudinal
Vibration direction compared with travel	Right angles	Parallel (along)
Pattern	Crests and troughs	Compressions and rarefactions
Examples	Ripples on water, rope waves	Sound in air, push-pull slinky
Wavelength	Crest to crest	Compression to compression

Model: a slinky. Side to side movement of the end gives transverse pulses. In and out movement along the spring gives longitudinal pulses.

Ripples, sound and measuring speed (P5.1j)

• Ripples on water surfaces model transverse waves: the surface moves up and down while the ripples move outwards.
• Sound in air is longitudinal: air vibrates back and forth along the direction of travel.

Wave	Method	Calculation
Ripples	Ripple tank. Find f (read the motor setting, or count ripples passing a mark in a timed interval). Measure across several wavelengths and divide by their number	v = f × λ
Ripples	Time a crest over a measured distance	v = distance ÷ time
Sound, echo	Time the echo from a wall at distance d	v = 2d ÷ t
Sound, two microphones	Timer between two microphones a measured distance apart	v = distance ÷ time
Sound, oscilloscope	Move a microphone until the trace shifts by one whole wave: that distance is λ	v = f × λ

For a short time interval, an electronic timer beats a hand-held stopwatch. Repeating and averaging improves reliability.

Evidence that the wave travels, not the medium (P5.1k)

• Ripples: a floating cork bobs up and down about the same spot while the ripples pass. It is not carried across the tank.
• Sound in air: you hear sound at a distance without a steady wind. Smoke or light dust near a speaker shakes to and fro about its position and does not drift away.
• Slinky: a pulse runs along the spring, but each coil returns to where it started.
• A wave transfers energy, not matter.

Model answers to learn

Describe how to measure the speed of ripples. Use a ripple tank with a dipper at a known frequency. Measure the length of several wavelengths on the screen with a ruler and divide by the number of waves to find one wavelength. Multiply the frequency by the wavelength to get the speed.

Describe how to measure the speed of sound in air. Choose a large flat wall and measure your distance to it. Clap sharply and time the echo, ideally electronically. Double the distance to the wall, divide by the time, and repeat to find an average.

Contrast transverse and longitudinal waves. A transverse wave vibrates perpendicular to the way it moves. A longitudinal wave vibrates along the way it moves. Give one example of each.

Describe evidence that the wave moves and not the medium. Name the object (a cork, smoke, a slinky coil), say how it moves (to and fro about a fixed place), and conclude that the medium does not travel with the wave.

Must-know distinctions

Often confused	Correct picture
Wavelength and amplitude	Wavelength is a length along the direction of travel. Amplitude is a height from the middle line.
Frequency and period	Frequency counts waves per second (Hz). Period is seconds per wave (s).
Transverse and longitudinal	At right angles to travel, or along travel
Distance to a wall and echo distance	Sound covers 2d before the echo returns
The wave and the medium	The wave moves on. The medium vibrates in place.

Quick self-test

1. Define the frequency of a wave.
2. A wave has a crest at +3.5 cm and a trough at −3.5 cm. State its amplitude.
3. A wave has frequency 50 Hz. Calculate its period.
4. A wave of frequency 120 Hz has wavelength 2.8 m. Calculate its speed.
5. A sound of frequency 850 Hz travels at 340 m/s. Calculate its wavelength.
6. Ripples travel at 0.45 m/s and have wavelength 1.5 cm. Calculate their frequency.
7. In a longitudinal wave, how does the direction of vibration compare with the direction of travel?
8. A clap is heard as an echo 1.0 s later from a wall 170 m away. Calculate the speed of sound.
9. A cork floats on water as ripples pass. What does it show about the water?
10. A sound has frequency 2.0 kHz and wavelength 0.17 m. Calculate the speed.
11. A wave has period 0.0040 s. Calculate its frequency.

Answers

1. The number of waves passing a point each second (or the number of vibrations per second).
2. 3.5 cm.
3. T = 1 ÷ 50 = 0.020 s.
4. v = 120 × 2.8 = 336 m/s.
5. λ = 340 ÷ 850 = 0.40 m.
6. λ = 0.015 m, so f = 0.45 ÷ 0.015 = 30 Hz.
7. They are parallel: the vibration is along the direction of travel.
8. The sound travels 2 × 170 = 340 m, so v = 340 ÷ 1.0 = 340 m/s.
9. It bobs up and down in about the same place, so the water vibrates but does not travel with the ripples. The wave travels.
10. f = 2000 Hz, so v = 2000 × 0.17 = 340 m/s.
11. f = 1 ÷ 0.0040 = 250 Hz.

Where marks are usually lost

• Writing the amplitude as the full crest-to-trough height.
• Leaving frequency in kHz or wavelength in cm or mm when substituting.
• Taking a period from a displacement-distance graph, or a wavelength from an oscilloscope trace.
• Defining frequency as “how fast the wave is”, which describes speed, not frequency.
• Saying a transverse wave moves “up and down” without relating that to the travel direction. Compare the two directions.
• Using d instead of 2d in an echo calculation.
• Saying the water or air “moves along with the wave” instead of vibrating about a fixed position.
• Describing a speed measurement without saying what is measured, how it is measured and what is calculated.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249). Specification Version 5.0, July 2026; first assessment 2018. Topic P5.1 Wave behaviour, statements P5.1a to P5.1e, P5.1j and P5.1k.