IGCSE Physics Topic 3 — Waves: Important Changes (2026–2028 Exams) Notes

Describe wave features and quantities using: wavefront, wavelength, frequency, crest (peak), trough, amplitude, wave speed.
Use the word perpendicular (not just “right angles”) for transverse waves.
Include effects of wavelength and gap size on diffraction and transmission through a gap.
Use electromagnetic radiation (not only “visible light”) when defining monochromatic.
State that all EM waves travel at the same high speed in a vacuum and approximately the same in air.
Harmful effects list now explicitly includes eye conditions (e.g. cataracts, retinal damage) for UV.
Satellite communications: include both microwaves and radio waves.

Term	Definition	Notes / Relations
Wavefront	Imaginary surface connecting all points on a wave that are in the same phase at a given instant.	Wavefronts are perpendicular to rays (direction of travel).
Wavelength (λ)	Distance between two consecutive points that are in phase.	Measured in metres (m); sets the spatial size of one cycle.
Frequency (f)	Number of complete cycles passing a point per second.	Unit: Hz; `f = 1/T` (T = period).
Crest (Peak)	Maximum positive displacement above the equilibrium position.	For transverse waves.
Trough	Maximum negative displacement below the equilibrium position.	For transverse waves.
Amplitude (A)	Maximum displacement of the wave from its equilibrium position.	Relates to intensity/strength; in longitudinal waves it is distance to compression/rarefaction.
Wave Speed (v)	Distance travelled by a wave per unit time.	`v = f × λ`

Direction of vibration is perpendicular to the direction of propagation.
Electromagnetic radiation: oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel (e.g., visible, radio, microwaves, X-rays, gamma).
Surface water waves: particles move in circular orbits; produces the up-and-down motion seen at the surface.
Seismic S-waves (secondary): transverse waves in Earth’s interior; do not travel through liquids.

Significant diffraction when wavelength ≈ gap size (or obstacle size).
As wavelength increases (for the same gap), spreading becomes more pronounced.
Smaller gaps (relative to λ) → stronger diffraction, more spreading, and more noticeable interference patterns.
Transmission depends on gap vs λ: when λ ≪ gap, most energy passes with little diffraction; when λ ≈ gap, spreading reduces central-beam intensity.

Monochromatic means electromagnetic radiation of a single frequency (single wavelength). Example: a laser emitting one specific wavelength.

Region	Effect
Microwaves	Internal heating of body cells.
Infra-red	Skin burns.
Ultraviolet	Damage to surface cells and eyes → skin cancer; eye conditions such as cataracts and retinal damage.
X-rays & Gamma	Cell mutation or damage to body cells.

Topic 3 — Waves: Important Changes (2026–2028 Exams)