What you'll learn
This revision guide covers the complete Waves topic as specified in the Pearson Edexcel International IGCSE Physics specification. You'll master the properties and behaviours of both mechanical and electromagnetic waves, understand wave calculations, and explore practical applications from seismic analysis to telecommunications. This topic typically accounts for 15-20% of your final examination.
Key terms and definitions
Wavelength (λ) — the distance between two consecutive points in phase on a wave, such as two adjacent crests, measured in metres (m)
Frequency (f) — the number of complete waves passing a point per second, measured in hertz (Hz)
Amplitude — the maximum displacement of a point on the wave from its rest position, measured in metres (m)
Transverse wave — a wave in which the oscillations are perpendicular to the direction of energy transfer; examples include electromagnetic waves and water waves
Longitudinal wave — a wave in which the oscillations are parallel to the direction of energy transfer; examples include sound waves and seismic P-waves
Refraction — the change in direction of a wave when it passes from one medium to another due to a change in wave speed
Normal — an imaginary line drawn perpendicular to a surface at the point where a wave ray meets that surface
Electromagnetic spectrum — the complete range of electromagnetic waves ordered by wavelength or frequency, from radio waves to gamma rays
Core concepts
Properties of waves
All waves transfer energy from one place to another without transferring matter. The key properties that describe any wave are:
- Wavelength (λ): measured from crest to crest or trough to trough
- Frequency (f): the number of waves per second
- Amplitude: affects the energy carried by the wave (higher amplitude = more energy)
- Period (T): the time taken for one complete wave to pass a point, where T = 1/f
The wave equation relates these quantities:
wave speed (v) = frequency (f) × wavelength (λ)
or v = fλ
Wave speed is measured in metres per second (m/s), frequency in hertz (Hz), and wavelength in metres (m).
Transverse and longitudinal waves
Transverse waves have oscillations perpendicular to the direction of energy transfer. Key examples include:
- All electromagnetic waves (light, radio, X-rays, etc.)
- Water waves (ripples on a pond)
- Seismic S-waves
- Waves on strings or springs when moved side-to-side
Transverse waves can be polarised — this means the oscillations can be restricted to one plane only. Only transverse waves can be polarised; longitudinal waves cannot.
Longitudinal waves have oscillations parallel to the direction of energy transfer. They consist of:
- Compressions: regions where particles are pushed together (high pressure)
- Rarefactions: regions where particles are spread apart (low pressure)
Key examples include:
- Sound waves in air, liquids and solids
- Seismic P-waves (primary waves)
- Pressure waves in springs
Reflection, refraction and diffraction
Reflection occurs when waves bounce off a surface. The law of reflection states:
- The angle of incidence = the angle of reflection
- Both angles are measured from the normal
- This applies to all wave types including light and sound
Refraction occurs when waves change direction as they pass from one medium to another due to a change in wave speed:
- When waves enter a denser medium, they slow down and bend towards the normal
- When waves enter a less dense medium, they speed up and bend away from the normal
- If a wave enters along the normal (at 90°), it does not change direction but may still change speed
Applications of refraction include:
- Lenses in glasses, cameras and microscopes
- Apparent depth of swimming pools (objects appear closer than they are)
- Prisms splitting white light into colours
Diffraction is the spreading out of waves when they pass through a gap or around an obstacle. Maximum diffraction occurs when:
- The gap width is approximately equal to the wavelength
- Longer wavelengths diffract more than shorter wavelengths
- This explains why long-wave radio signals can travel around hills while shorter-wave signals cannot
Sound waves
Sound waves are longitudinal waves that require a medium (solid, liquid or gas) to travel through — they cannot travel through a vacuum.
Key properties of sound:
- Speed in air at 20°C: approximately 330 m/s (increases with temperature)
- Speed in liquids: faster than in air (e.g., 1500 m/s in water)
- Speed in solids: fastest (e.g., 5000 m/s in steel)
- The denser and more rigid the medium, generally the faster sound travels
Pitch is determined by frequency:
- High frequency = high pitch
- Low frequency = low pitch
- Human hearing range: approximately 20 Hz to 20,000 Hz (20 kHz)
Loudness is determined by amplitude:
- Large amplitude = loud sound
- Small amplitude = quiet sound
Ultrasound refers to sound waves with frequencies above 20 kHz (above human hearing range). Applications include:
- Medical imaging (pregnancy scans, detecting tumours)
- Industrial quality control (detecting cracks in metals)
- Sonar and echo location
- Cleaning delicate instruments
Ultrasound works by reflecting off boundaries between different media. The time taken for reflections to return allows distance calculations using: distance = speed × time.
The electromagnetic spectrum
All electromagnetic waves:
- Are transverse waves
- Travel at the same speed in a vacuum: 3.0 × 10⁸ m/s (300,000 km/s)
- Can travel through a vacuum (unlike sound)
- Transfer energy
- Can be reflected, refracted and diffracted
The electromagnetic spectrum in order of increasing wavelength (or decreasing frequency):
- Gamma rays — shortest wavelength, highest frequency
- X-rays
- Ultraviolet
- Visible light (violet to red)
- Infrared
- Microwaves
- Radio waves — longest wavelength, lowest frequency
Hazards and uses of electromagnetic waves:
Radio waves:
- Uses: television, radio broadcasting, communications
- Hazards: none at normal exposure levels
Microwaves:
- Uses: satellite communications, mobile phones, cooking food
- Hazards: internal heating of body tissue; can damage living cells
Infrared:
- Uses: remote controls, thermal imaging, optical fibres, cooking
- Hazards: skin burns at high intensity
Visible light:
- Uses: vision, photography, illumination, optical fibres
- Hazards: bright lights can damage the retina
Ultraviolet:
- Uses: security marking, sterilisation, fluorescent lamps
- Hazards: skin cancer, premature skin aging, eye damage
X-rays:
- Uses: medical imaging, security scanners, treating cancer
- Hazards: ionising radiation can damage cells and cause cancer; mutations in DNA
Gamma rays:
- Uses: sterilising medical equipment, treating cancer (radiotherapy), detecting cancer
- Hazards: ionising radiation; most dangerous, can kill cells and cause cancer
Seismic waves
Earthquakes produce two main types of seismic waves:
P-waves (Primary waves):
- Longitudinal waves
- Travel through solids and liquids
- Fastest seismic waves (arrive first at detectors)
- Compressions and rarefactions in the direction of travel
S-waves (Secondary waves):
- Transverse waves
- Travel through solids only (cannot travel through liquids)
- Slower than P-waves (arrive second)
- Used to determine that the Earth's outer core is liquid
Seismologists use the detection of these waves at different locations to:
- Locate earthquake epicentres
- Determine the Earth's internal structure
- Provide early warnings for tsunamis
Worked examples
Example 1: Wave speed calculation
Question: A water wave has a frequency of 2.5 Hz and a wavelength of 1.2 m. Calculate the speed of the wave. (2 marks)
Solution:
- Use the wave equation: v = fλ (1 mark)
- v = 2.5 Hz × 1.2 m = 3.0 m/s (1 mark)
Answer: 3.0 m/s
Example 2: Electromagnetic spectrum
Question: (a) State one use of ultraviolet radiation. (1 mark) (b) Explain why ultraviolet radiation is hazardous to humans. (2 marks)
Solution: (a) Security marking / sterilisation / fluorescent lamps (any one) (1 mark)
(b) Ultraviolet radiation can damage DNA (1 mark) which can lead to skin cancer or premature aging (1 mark)
Example 3: Ultrasound distance calculation
Question: An ultrasound pulse is sent into the ground and reflects off a layer of rock. The pulse returns after 0.004 seconds. The speed of ultrasound in the ground is 4000 m/s. Calculate the depth of the rock layer. (3 marks)
Solution:
- Total distance travelled = speed × time = 4000 m/s × 0.004 s = 16 m (1 mark)
- This is the distance down and back (1 mark)
- Depth = 16 m ÷ 2 = 8 m (1 mark)
Answer: 8 m
Common mistakes and how to avoid them
Confusing wavelength and amplitude — Remember wavelength is the horizontal distance between crests; amplitude is the vertical distance from rest to crest. Always check which quantity the question asks for.
Forgetting to halve the distance in echo/ultrasound calculations — When a wave reflects and returns, it travels twice the actual distance to the reflector. Always divide the total distance by 2.
Stating sound travels fastest in gases — Sound actually travels fastest in solids, then liquids, then gases. Don't confuse this with electromagnetic waves, which travel at the same speed through all materials in a vacuum.
Mixing up angle of incidence and angle of refraction — The angle of incidence is in the first medium; the angle of refraction is in the second medium. Both are measured from the normal, not from the surface.
Writing "light years" as a unit of time — A light year is a unit of distance (the distance light travels in one year), not time. For IGCSE, stick to metres for distance.
Claiming longitudinal waves can be polarised — Only transverse waves can be polarised. If a question asks about polarisation, this confirms the wave is transverse.
Exam technique for "Waves"
Command word "Calculate" requires working — Always show your formula, substitution and final answer with units. Even if you use a calculator, write down the equation first to gain method marks if your final answer is incorrect.
Diagrams must include labels and the normal — When drawing reflection or refraction diagrams, always draw a dashed normal line perpendicular to the surface. Label incident ray, reflected/refracted ray, and mark angles clearly from the normal.
"Explain" questions need reasoning, not just facts — When asked to explain a hazard or use, state what happens AND why it matters. For example: "X-rays can ionise atoms (what) which damages DNA and can cause cancer (why it matters)."
Learn specific uses and hazards for each EM wave — Questions frequently ask for uses or dangers of particular parts of the electromagnetic spectrum. Generic answers like "communication" without specifying which waves won't score full marks.
Quick revision summary
Waves transfer energy without transferring matter. Use v = fλ for all wave calculations. Transverse waves oscillate perpendicular to energy transfer; longitudinal waves oscillate parallel. Sound requires a medium and travels fastest in solids. All electromagnetic waves travel at 3.0 × 10⁸ m/s in a vacuum, ordered by wavelength from gamma (shortest) to radio (longest). Higher frequency EM waves (UV, X-rays, gamma) are ionising and more hazardous. Reflection obeys angle of incidence = angle of reflection. Refraction occurs when waves change speed between media, bending towards the normal when slowing down.