IGCSE Physics: Waves (Cambridge 0625)

This guide covers Topic 3 Waves, for Cambridge IGCSE Physics 0625, 2026–2028 series. It marks which learning outcomes are Core (examined at grades C–G) and which are Supplement — required only for the Extended tier, needed for grades A*–C.

Where this fits in 0625

Waves is the third of the six 0625 topics. It introduces wave language (wavelength, frequency, amplitude, wave speed) that is then reused, unchanged, in Topic 6’s treatment of Space physics, through redshift and the cosmic microwave background. The general wave behaviours covered here — reflection, refraction and diffraction — are demonstrated with light and sound specifically, but the underlying ideas are the same ones examined generically in section 3.1, so a weak grasp of 3.1 tends to cost marks throughout the rest of the topic.

Syllabus coverage

CAMBRIDGE IGCSE PHYSICS 0625 — TOPIC 3 WAVES

• 3.1 General properties of waves (Core) — waves transfer energy without transferring matter; wave motion illustrated with ropes, springs and water waves; the terms wavefront, wavelength, frequency, crest, trough, amplitude and wave speed; the wave equation v = fλ; transverse waves (vibration at right angles to the direction of travel — electromagnetic radiation, water waves, seismic S-waves) and longitudinal waves (vibration parallel to the direction of travel — sound waves, seismic P-waves); reflection, refraction and diffraction, including their demonstration with a ripple tank; (Supplement) how wavelength and gap size affect diffraction through a gap, and how wavelength affects diffraction at an edge
• 3.2 Light, in four parts. Reflection (Core) — the terms normal, angle of incidence and angle of reflection; the image in a plane mirror (same size, same distance from the mirror, virtual); the law of reflection, angle of incidence = angle of reflection; (Supplement) constructions and calculations for plane-mirror reflection. Refraction (Core) — the terms normal, angle of incidence and angle of refraction; demonstrating refraction with transparent blocks; the critical angle; internal and total internal reflection; (Supplement) refractive index n = sin i / sin r, n = 1/sin c, and optical fibres in telecommunications. Thin lenses (Core) — the action of converging and diverging lenses on a parallel beam; focal length, principal axis and principal focus; ray diagrams for a real image formed by a converging lens; describing an image as enlarged/same size/diminished, upright/inverted, real/virtual; that a virtual image is formed by extrapolating diverging rays backwards; (Supplement) ray diagrams for a virtual image, the magnifying glass, and correcting long- and short-sightedness with lenses. Dispersion (Core) — dispersion of white light by a glass prism; the seven colours of the visible spectrum, in order of frequency and wavelength; (Supplement) monochromatic light
• 3.3 Electromagnetic spectrum (Core) — the main regions in order of frequency and wavelength; that all electromagnetic waves travel at the same high speed in a vacuum; typical uses of each region (radio, microwave, infrared, visible, ultraviolet, X-ray, gamma) and the harmful effects of excessive exposure to each; that communication with satellites is mainly by microwaves; (Supplement) the speed of electromagnetic waves in a vacuum, 3.0 × 10⁸ m/s; the systems that rely on specific regions (mobile phones and Wi-Fi on microwaves, Bluetooth on radio waves, optical fibres on visible/infrared light); digital vs analogue signals, and the benefits of digital signalling
• 3.4 Sound (Core) — sound produced by vibrating sources; the longitudinal nature of sound waves; the audible range, approximately 20 Hz to 20 000 Hz; that a medium is needed to transmit sound; the approximate speed of sound in air, 330–350 m/s; a method for determining the speed of sound in air; how amplitude and frequency affect loudness and pitch; echoes as reflected sound; ultrasound defined as above 20 kHz; (Supplement) compression and rarefaction; that sound travels faster in solids than liquids, and faster in liquids than gases; uses of ultrasound in non-destructive testing, medical scanning and sonar, including calculating depth/distance from time and wave speed

Refraction and refractive index

When light passes from one transparent material into another of different density (e.g. from air into glass, or glass into water), it changes speed, and — unless it hits the boundary exactly along the normal — this speed change also bends its direction. This bending is refraction. Light slows down and bends towards the normal on entering a denser material (e.g. air into glass), and speeds up and bends away from the normal on leaving it (e.g. glass into air).

Refractive index (n) is a number describing how strongly a material bends light — specifically, the ratio of the speed of light in a vacuum (or air) to the speed of light in the material. A larger refractive index means light slows down more, and bends more sharply, on entering that material. For a light ray entering the material from air, this is expressed as n = sin i / sin r, where i is the angle of incidence (in air) and r is the angle of refraction (in the material).

How to approach it

Section 3.1 is the foundation for the whole topic, so fix the wave vocabulary first — wavelength is measured between two identical points on consecutive waves (e.g. crest to crest), amplitude is measured from the middle position to a crest or trough, not crest to trough — and practise v = fλ in both directions (finding v given f and λ, and finding f given v and λ) since exam questions rearrange it freely. Keep transverse and longitudinal firmly separated by their examples: light and water waves are transverse, sound is longitudinal — if a question describes compressions and rarefactions, it is describing a longitudinal wave. For light, ray diagrams are worth as much practice as any calculation: know the three standard converging-lens diagrams (object beyond 2F, between F and 2F, inside F) and what each produces. For the electromagnetic spectrum, learn the order (radio, microwave, infrared, visible, ultraviolet, X-ray, gamma — by increasing frequency, decreasing wavelength) as a single sequence, since questions often ask you to place an unfamiliar use or hazard into the correct region by matching it to a property you do know. For sound, the ultrasound calculations (Supplement) use exactly the same speed = distance / time reasoning as elsewhere in the course, not the wave equation v = fλ — frequency and wavelength play no part. Treat depth = (speed × time) / 2 as a standard method to rehearse, halving the distance because the pulse travels out to the reflecting surface and back again before detection.

Examiner report insight

• Once the angle of incidence exceeds the critical angle, there is no refracted ray at all – only total internal reflection occurs. A common misconception treats total internal reflection as if a (weaker) refracted ray still escapes.
• A lens’s principal focus needs both parts of its definition together: it is the point where rays travelling parallel to the principal axis converge, after refraction by the lens – stating only one half (e.g. “where rays converge”) is an incomplete definition.
• In dispersion through a prism, blue light refracts more than red light – this is why a prism produces a spectrum, and is worth remembering as a specific fact rather than “light disperses” in general terms.

Source: Cambridge International, 0625 Physics Principal Examiner Report, June 2024 series, Papers 11, 12, 13, 21, 23, 33, 41 (verified 2026-09-02).

Official syllabus

Cambridge IGCSE Physics 0625 syllabus for 2026, 2027 and 2028 (Version 2, December 2025), Topic 3 verified against the PDF on 29 August 2026 — cambridgeinternational.org.