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 (taught to every candidate; a Core-only entry is eligible for grades C to G) and which are Supplement — examined only on the Extended papers (Papers 2 and 4). Extended candidates are eligible for grades A* to G, so Supplement content is needed for grades A* to B, and Cambridge advises candidates aiming for grades A* to C to be taught the Extended content.

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

This list summarises what the syllabus requires; it is not a list of what this page explains. The sections below, the revision notes and the practice sets teach it between them.

• 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 — (Supplement) 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.

Wave motion: ropes, springs and ripple tanks (3.1)

• Rope: move one end up and down. A wave travels along the rope, but each part of the rope only moves up and down, at right angles to the direction the wave travels: a transverse wave.
• Spring (slinky): push and pull one end along the spring’s length. Each coil moves back and forth parallel to the direction of travel: a longitudinal wave. Shaking the end from side to side sends a transverse wave instead. (Supplement) The longitudinal wave travels as compressions (coils close together) and rarefactions (coils spread out).
• Ripple tank: a vibrating bar dipping into shallow water makes straight ripples (a small dipper makes circular ones); a lamp above projects the wavefronts onto a screen or the floor below. A small floating object bobs up and down as the ripples pass but does not move along with them: the wave transfers energy, not water. Put a flat barrier in the water to show reflection; a sheet of glass or plastic that makes the water shallower slows the waves, shortens their wavelength and, if the boundary is at an angle, changes their direction (refraction); a gap between two barriers, or the edge of one barrier, shows diffraction.

Refraction experiments and total internal reflection (3.2.2)

Refraction through blocks. Put a rectangular glass (or Perspex) block on paper and draw round it. Shine a narrow ray from a ray box at the block at an angle, and mark the incoming and outgoing rays with crosses. Remove the block, join the points to draw the ray through the block, and draw the normal where the ray enters. Measure the angle of incidence and the angle of refraction with a protractor, and repeat for other angles. Repeat with blocks of other shapes, such as a triangular prism or a semicircular block.

Total internal reflection. Aim a ray through the curved side of a semicircular block towards the centre of its flat side: it crosses the curved surface along a normal, so it does not bend there. Increase the angle at which it meets the flat side from inside the block. At first the ray partly refracts out and partly reflects inside the block. At the critical angle the refracted ray runs along the flat surface; above it, all the light is reflected back inside: total internal reflection. Everyday examples: optical fibres and endoscopes, prisms in periscopes and binoculars, and the sparkle of a cut diamond.

Lenses: focal length and correcting sight (3.2.3)

A converging lens brings a parallel beam of light to a point, the principal focus. A diverging lens spreads a parallel beam out so that the rays appear to come from a principal focus on the same side as the incoming light. The focal length is the distance from the centre of the lens to the principal focus; a more strongly curved lens has a shorter focal length.

(Supplement) Correcting eyesight.

• Short sight: distant objects look blurred because the eye brings their light to a focus in front of the retina. A diverging lens spreads the rays out a little before they enter the eye, so they focus on the retina.
• Long sight: near objects look blurred because the eye would bring their light to a focus behind the retina. A converging lens bends the rays inwards before they enter the eye, so they focus on the retina.

Uses and hazards of electromagnetic waves (3.3)

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

Satellites. Communication with artificial satellites is mainly by microwaves. Some satellite phones use low-orbit satellites; other satellite phones and direct-broadcast satellite television use geostationary satellites, which stay above the same point on the Earth’s surface.

(Supplement) Why each system uses its wavelength. Mobile phones and wireless internet use microwaves, which can pass through some walls and need only a short aerial. Bluetooth uses radio waves, which pass through walls but are weakened in doing so. Optical fibres carry visible light or infrared, for cable television and high-speed broadband, because glass is transparent to them and these short wavelengths can carry data at high rates.

(Supplement) Digital and analogue signals. An analogue signal can take any value in a continuous range; a digital signal takes only a few set values (usually two, on and off, or 1 and 0). A sound can be sent either way. Digital signalling has two main benefits: data can be sent at a higher rate, and a signal that has picked up noise can be regenerated accurately (cleaned back to exact 1s and 0s) at each repeater along the way, so it can be sent further without losing quality. Amplifying an analogue signal amplifies its noise too.

Uses of ultrasound (3.4) (Supplement)

• Non-destructive testing: pulses sent into a metal part are reflected by the far side, and also by any crack, which returns an echo earlier than expected, without damaging the part.
• Medical scanning of soft tissue: pulses are partly reflected at each boundary between tissues; the echo times build an image (for example of a fetus) without using ionising radiation.
• Sonar: a ship times the echo from the sea bed or a shoal of fish. Sound travels at about 1500 m/s in sea water; if the echo returns after 0.40 s, depth = (1500 × 0.40) ÷ 2 = 300 m.

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.