IGCSE Physics: Waves — Revision Notes (Cambridge 0625)

Condensed for the final weeks. For the full explanation, use the Waves study guide.

3.1 General wave properties — the foundation

Waves transfer energy without transferring matter. Key terms: wavefront, wavelength (crest to crest, or any two identical points on consecutive waves — never crest to trough), frequency, amplitude (middle position to crest/trough, not crest to trough), wave speed. Wave equation: v = fλ — practise rearranging both ways.

Wave type	Vibration direction	Examples
Transverse	At right angles to travel direction	Electromagnetic radiation (including light), water waves, seismic S-waves
Longitudinal	Parallel to travel direction	Sound waves, seismic P-waves

Wave motion is shown by a rope moved up and down (transverse), a spring pushed and pulled along its length (longitudinal; Supplement: compressions and rarefactions) and ripples in a ripple tank, which also shows reflection (barrier), refraction (shallower water slows the waves) and diffraction (gap or edge).

(Supplement) If a question mentions compressions and rarefactions, it is describing a longitudinal wave. (Supplement) Diffraction through a gap increases as wavelength increases relative to gap size; diffraction also occurs at an edge, more noticeably for longer wavelengths.

3.2 Light — reflection

Terms: normal, angle of incidence, angle of reflection. Plane mirror image: same size, same distance from mirror, virtual. Law of reflection: angle of incidence = angle of reflection.

3.2 Light — refraction

Light entering a denser material slows down and bends towards the normal; leaving it, light speeds up and bends away from the normal. Critical angle: beyond it, no refracted ray exists at all — only total internal reflection. Experiment: trace a ray from a ray box through glass blocks of different shapes, mark it with crosses, draw the normal and measure the angles. Total internal reflection: a ray aimed at the centre of the flat side of a semicircular block, with the angle increased past the critical angle. (Supplement) refractive index n = sin i / sin r = 1/sin c. Optical fibres use total internal reflection for telecommunications.

3.2 Light — thin lenses

Converging lens: parallel rays meet at the principal focus — defined as where rays parallel to the principal axis converge after refraction; both halves of this definition are required. Know the three standard ray-diagram cases: object beyond 2F (real, inverted, diminished), between F and 2F (real, inverted, enlarged), inside F (virtual, upright, enlarged — magnifying glass). A virtual image is formed by extrapolating diverging rays backwards. Focal length = distance from the centre of the lens to the principal focus. A diverging lens spreads a parallel beam as if from a principal focus on the incoming side. (Supplement) Short sight (distant objects blurred, light focused in front of the retina): corrected with a diverging lens. Long sight (near objects blurred, focus behind the retina): corrected with a converging lens.

3.2 Light — dispersion

White light disperses through a glass prism into the seven-colour visible spectrum. Blue light refracts more than red light — the specific fact that explains why a prism produces a spectrum at all, not just that “light disperses.”

3.3 Electromagnetic spectrum

Order by increasing frequency, decreasing wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. All travel at the same high speed in a vacuum (Supplement: 3.0 × 10⁸ m/s). Learn this sequence as one chain — questions often place an unfamiliar use or hazard by matching it to a property (frequency/energy level) you already know. Mobile phones use microwaves; optical fibres use visible/infrared light. (Supplement) Wireless internet (Wi-Fi) also uses microwaves and Bluetooth uses radio waves; digital signals resist noise better than analogue.

Region	Uses	Harm (excessive exposure)
Radio	radio and TV transmissions, astronomy, RFID	none listed in the syllabus
Microwave	satellite TV, mobile phones, microwave ovens	internal heating of body cells
Infrared	electric grills, remote controls, intruder alarms, thermal imaging, optical fibres	skin burns
Visible	vision, photography, illumination	none listed in the syllabus
Ultraviolet	security marking, detecting fake bank notes, sterilising water	damage to surface cells and eyes: skin cancer, eye conditions
X-ray	medical scanning, security scanners	mutation or damage to cells
Gamma	sterilising food and medical equipment, detecting and treating cancer	mutation or damage to cells

Satellite communication is mainly by microwaves: some satellite phones use low-orbit satellites; other satellite phones and direct-broadcast satellite TV use geostationary satellites. (Supplement) Microwaves pass through some walls and need only a short aerial; radio waves (Bluetooth) pass through walls but are weakened; glass is transparent to visible light and some infrared, which carry high data rates.

(Supplement) Analogue signals vary continuously; digital signals take only set values (1 and 0). Sound can be sent as either. Digital benefits: a higher data rate, and a longer range because the signal can be regenerated accurately.

3.4 Sound

Produced by vibrating sources; longitudinal; audible range approximately 20 Hz to 20 000 Hz; needs a medium (does not travel through a vacuum); speed in air approximately 330–350 m/s. Amplitude affects loudness; frequency affects pitch. Echoes are reflected sound. Ultrasound: above 20 kHz. (Supplement) sound travels faster in solids than liquids, faster in liquids than gases. Ultrasound depth/distance calculations use speed = distance/time, NOT v = fλ — frequency and wavelength play no part. Use depth = (speed × time) / 2, halving because the pulse travels out and back. Other uses of ultrasound (Supplement): non-destructive testing (echoes from cracks in metal) and medical scanning of soft tissue (echoes from tissue boundaries).

Why 3.1 underpins everything else in this topic

Section 3.1 introduces wave language generically – wavelength, frequency, amplitude, wave speed, reflection, refraction, diffraction – before the rest of the topic demonstrates these same ideas using light and sound specifically. A weak grasp of the generic vocabulary in 3.1 tends to resurface as lost marks throughout 3.2, 3.3 and 3.4, since a question on light refraction or ultrasound distance still assumes the underlying wave concepts from 3.1 are secure. This vocabulary is also reused unchanged much later in the course, in Topic 6’s treatment of Space physics through redshift and the cosmic microwave background, so treating 3.1 as foundational rather than a self-contained block pays off well beyond this topic alone.

Exam traps

• Measuring amplitude crest-to-trough instead of middle-to-crest.
• Treating total internal reflection as if a weaker refracted ray still escapes past the critical angle.
• Stating only half the principal focus definition.
• Using v = fλ for an ultrasound distance calculation instead of speed = distance/time.
• Forgetting blue light refracts more than red light in dispersion.

Self-test

1. State the wave equation and rearrange it to find frequency given speed and wavelength.
2. Name two examples each of transverse and longitudinal waves.
3. What happens to the refracted ray once the angle of incidence exceeds the critical angle?
4. State both parts of the definition of a lens’s principal focus.
5. List the electromagnetic spectrum in order of increasing frequency.
6. Why can’t ultrasound distance calculations use v = fλ?

Answers: 1. v = fλ; rearranged, f = v/λ. 2. Transverse: light, water waves (or seismic S-waves). Longitudinal: sound (or seismic P-waves). 3. There is no refracted ray at all — only total internal reflection occurs. 4. The point where rays travelling parallel to the principal axis converge, after refraction by the lens. 5. Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. 6. Because frequency and wavelength play no part in the calculation — it uses speed = distance/time (and halves the result, since the pulse travels to the surface and back).

Official syllabus

Cambridge IGCSE Physics 0625 syllabus for 2026, 2027 and 2028 (Version 2, December 2025), Topic 3 — the same source already cited by the Waves study guide, which first reproduced this topic’s content from it. Verified 2026-09-06.