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	Light, water waves, seismic S-waves
Longitudinal	Parallel to travel direction	Sound, seismic P-waves

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. (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.

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/Wi-Fi use microwaves; Bluetooth uses radio waves; optical fibres use visible/infrared light. (Supplement) digital signals resist noise better than analogue.

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.

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.