OxfordAQA International GCSE Physics: Wave properties, sound and ultrasound (9203) – Revision Notes

These notes condense sections 3.3.1 (general properties of waves) and 3.3.3 (sound and ultrasound) of the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards. Parts 3.3.3 d to h are marked “P” in the specification, for full Physics. For full explanations read the study guide, then test yourself with the practice questions.

Other useful pages: the course hub, the printable checklist, the 10-minute diagnostics and the exam preparation page.

Definitions to learn word for word

Term	Definition
Wave	A disturbance from a vibrating source that carries energy and information along its path while the material itself is not carried along
Transverse wave	Oscillations at right angles to the direction energy moves
Longitudinal wave	Vibrations along the line of energy travel
Compression	Region where the particles or coils are squashed together
Rarefaction	Region where the particles or coils are spread apart
Amplitude	Largest displacement of a point from its undisturbed position
Wavelength, λ	Span of one full wave, matching point to matching point
Frequency, f	Waves going past a point each second (hertz)
Period, T	Time taken by one complete wave (seconds)
Wavefront	Line joining points at the same stage of oscillation
Ultrasound	Sound at frequencies higher than humans can hear (above 20 000 Hz)

Wave types and examples

Type	Examples
Transverse	Electromagnetic waves, water waves, a rope shaken sideways
Longitudinal	Sound waves, a pushed and pulled spring

• Mechanical waves can be transverse or longitudinal.
• Sound needs a medium and cannot cross a vacuum.
• A floating cork bobs up and down but does not travel with the ripples. Energy moves, matter does not.

Behaviour at a boundary

A wave meeting a boundary between two materials can be reflected, transmitted, absorbed, or any combination of these.

Effect	What happens
Reflection	Wave bounces back into the first material
Transmission	Wave passes into the second material
Absorption	Wave energy is transferred to the material
Refraction	Wave speed changes, so wavelength changes and direction can change. Frequency stays the same
Diffraction	Waves spread through a narrow gap or round an edge

Diffraction rule: it is appreciable only when the wavelength is comparable to the size of the gap or obstacle.

Gap compared with wavelength	Result
Much wider	Very little spreading
About the same	Strong spreading
Much narrower	Weak waves emerge, spreading widely

Applications named in the specification: reducing diffraction in optical instruments, ultrasound in medicine, radio wave reception.

Key equations

Quantity	Rule	Unit of each symbol
Wave speed	v = fλ	speed m/s, frequency Hz, wavelength m
Period	T = 1/f	s, Hz
Distance	s = vt	m, m/s, s

Rearrangements: f = v/λ, λ = v/f, t = s/v.

Method for v = fλ

1. Write the known values with units.
2. Convert units first (kHz to Hz, cm to m, ms to s).
3. Rearrange for the unknown, then substitute.
4. Give the answer with a unit and sensible significant figures.

Reminder. A wave of frequency 1200 Hz has a wavelength of 0.28 m. The speed is 1200 x 0.28 = 336 m/s, which is close to the speed of sound in air.

Reminder. A mains-frequency vibration of 50 Hz has a period T = 1/50 = 0.020 s. A period of 0.004 s means a frequency of 1/0.004 = 250 Hz.

Sound

• Sound is a longitudinal wave. The source vibrates, making compressions and rarefactions in the medium.
• Human hearing: about 20 Hz to 20 000 Hz.
• The structure of the ear is not required.
• Pitch depends on the frequency of the source. Loudness is related to the amplitude.
• Sound can be reflected (echoes) and diffracted.

Change to the source	Oscilloscope trace
Louder	Taller wave, same number of cycles
Quieter	Smaller wave
Higher pitch	More cycles across the screen, shorter period
Lower pitch	Fewer cycles, longer period

Method for a frequency from a trace

1. Count how many divisions one complete cycle takes.
2. Multiply by the time base to get T in seconds.
3. Use f = 1/T.

Reminder. If 4 divisions at 0.5 ms per division make one cycle, T is 2.0 ms and f is 500 Hz.

Ultrasound (P)

• Frequency above 20 000 Hz, beyond the range of human hearing.
• Produced by electronic systems: an electrical signal makes a transducer vibrate at high frequency. The same device can detect returning pulses.
• At a boundary between two media it is partially reflected. Some carries on, some returns.
• The time for a reflection to reach the detector gives the distance to the boundary.

Method for echo and ultrasound distances

1. Read the round-trip time from the question or trace.
2. Convert to seconds (1 µs = 10^-6 s, 1 ms = 10^-3 s).
3. Halve it, because the pulse travels there and back.
4. Use s = vt.
5. For the gap between two boundaries, find each distance and subtract, or halve the difference in echo times first.

Reminder. An echo from a wall returns after 0.50 s in air at 340 m/s. The wall is 340 x 0.25 = 85 m away.

Reminder. A pulse in tissue (1500 m/s) returns 50 µs after being sent. The one-way time is 25 µs, so s = 1500 x 25 x 10^-6 = 0.0375 m, about 3.8 cm.

Reminder. Wavelength of 2.0 MHz ultrasound in tissue at 1500 m/s: 1500 / 2 000 000 = 0.00075 m (0.75 mm). A wavelength this small means very little diffraction, so the beam stays narrow.

Medical uses of ultrasound (P)

Use	Key points
Prenatal scanning	Pulses reflect from boundaries between tissues and fluid. Reflection times build an image. Non-ionising
Kidney stones	Focused, high-intensity ultrasound breaks the stone into small pieces that pass out in urine

Exam wording to practise

• “Explain why an echo is heard”: sound is reflected by a hard surface and arrives later than the original sound because it has travelled further.
• “Explain what happens to a wave entering a different medium”: the speed changes, so the wavelength changes, the frequency stays the same, and the direction can change if the wave arrives at an angle.
• “Describe how ultrasound finds a boundary”: a pulse is sent, part of it reflects at the boundary, the detector records the time, and the distance is found from s = vt using half the time.
• “Compare two traces”: say which has the larger amplitude (louder) and which has the higher frequency (higher pitch), and give the reason for each.

Must-know distinctions

• Pitch vs loudness: pitch is frequency, loudness is amplitude.
• Frequency vs period: one is the reciprocal of the other.
• Reflection vs refraction: reflection sends the wave back, refraction changes its speed.
• Reflection vs absorption: reflected energy stays in the wave, absorbed energy goes into the material.
• Sound vs ultrasound: same type of wave, but ultrasound is above the hearing range.
• Wavelength vs amplitude: wavelength is along the direction of travel, amplitude is the size of the oscillation.

Quick self-test

1. Say how the oscillation direction differs between transverse and longitudinal waves.
2. Name one example of each.
3. A wave has a frequency of 150 Hz and a wavelength of 2.2 m. Find its speed.
4. What is the period of a 250 Hz sound?
5. A note is played louder but at the same pitch. What changes on the oscilloscope trace?
6. A sound takes 0.60 s to travel to a wall and back at 340 m/s. Find the distance to the wall.
7. Why does a 0.25 m wavelength sound diffract strongly through a 0.30 m gap but a 0.5 mm ultrasound wave does not?
8. Calculate the wavelength of a 40 kHz ultrasound wave in air at 340 m/s.
9. State what happens to the frequency and wavelength when a wave slows down on entering shallow water.
10. Give two reasons ultrasound suits prenatal scanning.

Answers

1. Transverse: the oscillation is at right angles to the energy flow. Longitudinal: it is along the energy flow.
2. Transverse: light or water waves. Longitudinal: sound.
3. v = 150 x 2.2 = 330 m/s.
4. T = 1/250 = 0.0040 s.
5. The amplitude (height) increases. The period stays the same.
6. One-way time 0.30 s, so s = 340 x 0.30 = 102 m, about 100 m.
7. Diffraction is appreciable when the wavelength is comparable to the gap. 0.25 m is close to 0.30 m. 0.5 mm is far smaller than 0.30 m.
8. λ = 340 / 40 000 = 0.0085 m (8.5 mm).
9. Frequency is unchanged. Wavelength decreases (the speed decreases).
10. It is non-ionising, and tissue boundaries partially reflect it, so reflections can build an image.

Where marks are usually lost

• Writing “the wave carries the particles along” instead of “transfers energy without transferring matter”.
• Using the full echo time in s = vt.
• Mixing up pitch and loudness, or writing “volume” and “pitch” without the matching physical quantity (amplitude, frequency).
• Not converting µs, ms, kHz or MHz.
• Saying diffraction depends only on the gap size, with no comparison to the wavelength.
• Saying refraction changes frequency.
• Describing a longitudinal wave without naming compressions and rarefactions.
• Giving an answer with no unit, or with far too many significant figures.
• Writing “sound bounces off” when asked to explain partial reflection at a boundary: say some is reflected and some transmitted.

Official syllabus

OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.3.1 and 3.3.3.