AQA GCSE Physics 8463: Waves – Study Guide

This study guide teaches Topic 6, Waves (sections 4.6.1 to 4.6.3), of the AQA GCSE Physics (8463) specification, for first teaching 2016 with exams from June 2018 (version 1.1). Waves is examined on Paper 2 (1 hour 45 minutes, 100 marks, 50% of the GCSE) with Forces, Magnetism and electromagnetism, and Space physics. Both tiers sit this topic; “(HT only)” statements are labelled Higher tier only below. Exams run every May/June.

Use it with the Waves revision notes and the Waves practice questions. The course hub is AQA GCSE Physics and the printable checklist lists every statement. Find your gaps with a free diagnostic.

What this topic covers

Spec	What you must be able to do	Tier
4.6.1.1–4.6.1.2	Transverse and longitudinal waves; amplitude, wavelength, frequency, period; T = 1/f and v = fλ; Required practical 8	Both
4.6.1.3	Reflection, absorption, transmission; ray diagrams; Required practical 9	Both
4.6.1.4	Sound waves in solids and the ear; limits of human hearing	Higher tier only
4.6.1.5	Ultrasound, seismic P- and S-waves, echo sounding	Higher tier only
4.6.2.1–4.6.2.4	EM spectrum, refraction, Required practical 10, hazards, dose, uses. Wave fronts, radio circuits and “why suitable” are Higher tier only	Both / HT
4.6.2.5–4.6.2.6	Convex and concave lenses, magnification; colour, filters, specular and diffuse reflection	Both
4.6.3.1–4.6.3.2	Infrared emission and absorption, black bodies; radiation balance (HT only)	Both / HT

Transverse and longitudinal waves (4.6.1.1)

• In a transverse wave the oscillations are perpendicular to the direction of energy transfer. Ripples on water are transverse, and so are all electromagnetic waves.
• In a longitudinal wave the oscillations are parallel to the direction of energy transfer. They have compressions (particles close together) and rarefactions (particles spread out). Sound in air is longitudinal.

Evidence that the wave travels, not the medium. A cork on a pond bobs up and down as ripples pass but does not move along with them. Air is not blown from a loudspeaker to your ear: the particles vibrate about fixed positions.

Describing waves (4.6.1.2)

• Amplitude: the maximum displacement of a point on a wave from its undisturbed position (middle line to crest).
• Wavelength (λ): the distance from a point on one wave to the equivalent point on the adjacent wave.
• Frequency (f): the number of waves passing a point each second, in hertz (Hz).
• Period (T): the time for one wave, in seconds.
• Wave speed (v): the speed at which energy is transferred through the medium.

T = 1/f is on the equation sheet. v = f λ (m/s, Hz, m) is not: you must recall it.

Worked example 1. A wave on a rope has frequency 4.0 Hz and wavelength 0.75 m. Find its speed and period.

v = f λ = 4.0 × 0.75 = 3.0 m/s
T = 1/f = 1/4.0 = 0.25 s

Measuring the speed of sound in air

Stand a measured distance from a large wall and clap. Time from the clap to the echo. The sound travels there and back, so double the distance.

Worked example 2. You stand 99 m from a wall. The echo returns 0.60 s after the clap.

distance travelled = 2 × 99 = 198 m
v = 198 / 0.60 = 330 m/s

Reaction time is the main error: use a large distance and repeat.

Required practical 8: waves in a ripple tank and in a solid

Ripple tank. A lamp above the tank casts the waves onto paper below. Measure across 10 waves and divide by 10: 10 waves over 18 cm gives λ = 0.018 m. Read the frequency from the signal generator, or count waves passing a point in 10 s. At 8.0 Hz, v = 8.0 × 0.018 = 0.144 m/s.

Waves on a stretched string. A vibration generator drives a string over a pulley with hanging masses. Adjust the frequency until steady loops form; each loop is half a wavelength. Four loops over 1.20 m gives λ = 0.60 m; at 40 Hz, v = 40 × 0.60 = 24 m/s.

Measuring over several waves reduces the uncertainty in each reading.

Sound changing medium

When sound passes from one medium to another, its frequency stays the same (it is set by the source). Speed changes, so wavelength changes in proportion.

Worked example 3. A 500 Hz sound passes from air (330 m/s) into water (1500 m/s).

λ in air   = 330 / 500  = 0.66 m
λ in water = 1500 / 500 = 3.0 m

Reflection, absorption and transmission (4.6.1.3)

At a boundary between two materials, a wave can be reflected, absorbed or transmitted, often some of each. In a ray diagram of reflection, draw the normal at 90° to the surface; the angle of incidence equals the angle of reflection, both measured from the normal.

Required practical 9: reflection and refraction of light

Shine a ray-box ray at different surfaces, and into blocks of different substances, on paper. Mark the rays with crosses, join them with a ruler, draw the normal and measure angles with a protractor. Work in a darkened room.

Sound in solids and hearing (4.6.1.4) – Higher tier only

Sound waves can make solids vibrate, and vibrating solids produce sound. In the ear, sound makes the ear drum and other parts vibrate, giving the sensation of sound. This conversion only works over a limited frequency range, which limits human hearing. Normal human hearing is 20 Hz to 20 kHz.

Detection and exploration (4.6.1.5) – Higher tier only

Ultrasound has a frequency above 20 kHz. It is partly reflected at each boundary between two media. The time for the echo to return gives the distance to the boundary, which is used in medical and industrial imaging.

Worked example 4. Ultrasound travels at 1500 m/s in soft tissue. An echo returns 80 μs after the pulse is sent.

total distance = 1500 × 80 × 10⁻⁶ = 0.12 m
depth of boundary = 0.12 / 2 = 0.060 m = 6.0 cm

Echo sounding uses high-frequency sound to find objects in deep water and measure depth. A pulse returning after 0.40 s in seawater at 1500 m/s means a depth of 1500 × 0.40 / 2 = 300 m.

Seismic waves come from earthquakes.

• P-waves are longitudinal and travel through solids and liquids, at different speeds in each.
• S-waves are transverse and cannot travel through a liquid.

S-waves are not detected on the far side of the Earth from an earthquake, so part of the core must be liquid. P-waves change speed and direction at the core boundary, which shows its size. This gave new evidence about parts of the Earth that cannot be observed directly.

Electromagnetic waves (4.6.2.1–4.6.2.4)

EM waves are transverse and transfer energy from a source to an absorber. They form a continuous spectrum, and all travel at the same speed in a vacuum or air. From long wavelength (low frequency) to short wavelength (high frequency):

radio → microwave → infrared → visible (red to violet) → ultraviolet → X-rays → gamma rays

Your eyes detect only visible light. Sunlight warming your skin is energy transfer by EM waves.

Worked example 5. A microwave oven uses 2.5 GHz. Taking the speed of EM waves as 3.0 × 10⁸ m/s:

λ = v / f = 3.0 × 10⁸ / 2.5 × 10⁹ = 0.12 m

Refraction. A wave crossing a boundary at an angle changes direction. Light entering glass bends towards the normal; leaving glass it bends away from the normal. A ray along the normal does not change direction.

Higher tier only. Different substances may absorb, transmit, refract or reflect EM waves in ways that vary with wavelength. Refraction happens because the wave’s speed changes: light slows down in glass. In a wave front diagram, the end of each wave front that enters the slower medium first slows first while the rest moves on faster, so the wave front turns. Wave fronts are closer together in the slower medium (shorter wavelength).

Higher tier only. Oscillations in electrical circuits can produce radio waves. Absorbed radio waves can create an alternating current of the same frequency.

Origins and hazards. Changes in atoms and nuclei can generate or absorb EM waves over a wide frequency range. Gamma rays come from changes in the nucleus. Ultraviolet can make skin age prematurely and increases the risk of skin cancer. X-rays and gamma rays are ionising and can cause gene mutation and cancer. Radiation dose, in sieverts (Sv), measures the risk of harm; 1000 mSv = 1 Sv, so 2.4 mSv = 0.0024 Sv. You need not recall the unit, but may have to draw conclusions from dose data.

Uses.

Group	Uses	Why suitable (Higher tier only)
Radio	Television and radio	Produced and detected by circuits; travel long distances
Microwaves	Satellite communications, cooking	Pass through the atmosphere; absorbed by water in food
Infrared	Heaters, cooking, infrared cameras	Absorbed by surfaces, heating them; warm objects emit it
Visible light	Fibre optic communications	Travels along glass fibres
Ultraviolet	Energy efficient lamps, sun tanning	Makes some materials glow
X-rays, gamma	Medical imaging and treatment	Pass through soft tissue; high energy kills cells

Required practical 10: infrared and surfaces

Fill a Leslie cube (faces of different finish) with hot water. Hold an infrared detector the same distance from each face and record the reading. Matt black emits most; shiny silver least.

Lenses and visible light (4.6.2.5–4.6.2.6)

A lens forms an image by refraction. A convex lens brings parallel rays to a focus at the principal focus; the distance from lens to principal focus is the focal length. A convex lens can form a real image (rays actually meet) or a virtual one (rays only appear to come from it). A concave lens always gives a virtual image.

magnification = image height / object height (on the equation sheet; no units; use mm or cm for both heights)

Worked example 6. An object 2.0 cm tall gives an image 7.0 cm tall. Magnification = 7.0 / 2.0 = 3.5.

Colour. Each colour has its own narrow band of wavelength and frequency.

• Specular reflection: from a smooth surface, in a single direction. Diffuse reflection: from a rough surface, scattered.
• An opaque object’s colour is the wavelengths it reflects most strongly; the rest are absorbed. Reflects all equally: white. Absorbs all: black.
• A filter transmits some wavelengths and absorbs the rest. Objects that transmit light are transparent or translucent.

A red object seen through a green filter looks black: the object reflects only red, and the green filter absorbs red.

Black body radiation (4.6.3)

All objects, at any temperature, emit and absorb infrared radiation. The hotter the object, the more it radiates in a given time. The intensity and wavelength distribution of the emission depend on temperature: a hotter object emits more at every wavelength, with more at shorter wavelengths.

A perfect black body absorbs all radiation that falls on it; it reflects and transmits none. A good absorber is also a good emitter, so a perfect black body is the best possible emitter.

Higher tier only. An object at constant temperature absorbs radiation at the same rate as it emits it. If it absorbs faster than it emits, its temperature rises; if it emits faster, it cools. The Earth’s temperature depends on the rates it absorbs and emits radiation and how much is reflected into space. If less radiation escapes while absorption is unchanged, the temperature rises until a new balance is reached.

Common errors

• Measuring amplitude from crest to trough, which doubles it.
• Forgetting to halve the distance in echo, ultrasound and sonar questions.
• Saying frequency changes in a new medium. Speed and wavelength change.
• Leaving GHz, MHz, kHz or μs unconverted before using v = f λ.
• Measuring angles from the surface instead of the normal.
• Saying a black body “only absorbs”. It is also the best emitter.

Official syllabus

AQA GCSE Physics (8463) specification, for first teaching 2016, exams from June 2018, version 1.1 (30 September 2019), published by AQA – section 4.6 Waves.