Waves

Waves is the third topic in Cambridge IGCSE Physics (0625) and one of the heaviest. Expect 8 to 12 marks across Paper 1 or 2, plus a ray-diagram or sound question on almost every Paper 3 or 4. It is also the most drawable topic: rulers and pencils earn real marks here.

What does the Waves topic include in 0625?

Six strands: general wave properties ($v = f\lambda$, amplitude, wavefronts); transverse versus longitudinal waves; reflection, refraction and diffraction; light, including lenses and total internal reflection; the electromagnetic spectrum; and sound. Extended candidates add refractive index calculations ($n = \dfrac{\sin i}{\sin r}$), the critical angle relationship, thin-lens ray diagrams for virtual images, and communication applications of EM waves.

One number anchors the whole topic: every electromagnetic wave travels at $3.0 \times 10^{8}\ \text{m/s}$ in a vacuum. Examiners test that fact directly almost every session.

Why do students lose marks on Waves?

Diagrams, mostly. Ray diagrams drawn freehand, missing arrows, or normals omitted at the boundary cost easy marks. The angle of incidence is measured from the normal, not the surface, and that single error sinks thousands of scripts each year. On calculations, students mix units: a wavelength given in centimetres with speed in m/s needs converting before $v = f\lambda$. On the EM spectrum, candidates memorise the order but reverse it under pressure; learn it once with frequency increasing and stick to that direction.

Sound questions look easy and are not. Echo calculations involve a doubled distance, and roughly half of all candidates forget to halve or double at the right moment.

How should you revise it?

Drill three skills separately. First, the $v = f\lambda$ triangle with unit conversions until substitution is automatic. Second, ruler-and-protractor ray diagrams: plane mirror, glass block, converging lens at the three standard object positions. Third, write the EM spectrum order, one use and one hazard per region, from memory in under two minutes. Then sit ten past-paper wave questions against the mark scheme.

Our tutors run waves as a drawing-first topic in 1.5-hour classes, because Cambridge awards method marks for construction lines that most students never claim. That is 1-to-1 at RM80/hr, one tutor on one student. Your first hour is a free taught trial, and many parents use it to test exactly this topic. Book it on WhatsApp.

How Waves Is Assessed Across the Papers

Waves is one of the heaviest topics, so it spreads across the whole assessment. Papers 1 (Core) and 2 (Extended) carry several multiple-choice items: the wave equation $v = f\lambda$, the order of the electromagnetic spectrum, reflection angles, and properties of sound. Papers 3 (Core) and 4 (Extended) almost always include a ray-diagram question and a wave calculation worth several marks. The Core and Extended split is wide here. The Supplement tier adds refractive index $\left(n = \dfrac{\sin i}{\sin r}\right)$, the critical angle relationship, thin-lens ray diagrams for virtual images, and the uses of electromagnetic waves in communication. Those Extended-only rows reward neat constructions. Papers 5 (Practical) and 6 (Alternative to Practical) suit this topic perfectly: tracing rays through a glass block, finding the refractive index from a graph of $\sin i$ against $\sin r$, and measuring the speed of sound by echo are recurring set-ups.

A Worked Example That Shows the Method

A ray of light travels from air into a glass block. The angle of incidence is $40^\circ$ and the refractive index of the glass is $1.5$. Calculate the angle of refraction. [3]

Worked solution:

$n = \dfrac{\sin i}{\sin r} \quad\Rightarrow\quad \sin r = \dfrac{\sin i}{n}$

$\sin r = \dfrac{\sin 40^\circ}{1.5} = \dfrac{0.643}{1.5} = 0.429$

$r = \sin^{-1}(0.429) = 25.4^\circ$

So the angle of refraction is $25^\circ$ (2 significant figures), measured from the normal. The ray bends towards the normal as it enters the denser glass, which is the sense check. A frequent slip is to measure both angles from the surface rather than the normal, which silently inverts the whole calculation. Another is to leave the answer as $\sin r = 0.429$ without taking the inverse sine, so the final step never appears and the answer mark is lost. Always finish with the angle itself, stated in degrees and measured from the normal.

Mark scheme:

• M1: $n = \dfrac{\sin i}{\sin r}$ rearranged to $\sin r = \dfrac{\sin i}{n}$
• A1: $\sin r = 0.429$ (or equivalent)
• B1: $r = 25^\circ$ stated as measured from the normal

Keep the full physics equations list beside you while you practise, and follow our calculation-question guide for the equation, substitute and rearrange routine that protects method marks.