IB Physics: Interference & the Double-Slit Experiment FAQ

It's tested through calculation questions using the double-slit formula, qualitative explanations of constructive/destructive interference, and data-analysis questions on fringe patterns. According to the IB Physics guide (first exams 2025), this sits under Topic 9 (Wave Phenomena) at HL, with simpler two-source interference ideas appearing at SL too.

It's Young's classic demonstration that light behaves as a wave: coherent light passes through two closely-spaced slits and produces alternating bright and dark fringes on a screen. The pattern only forms because the two slits act as coherent sources with a constant phase relationship — remove that coherence and the fringes wash out.

Fringe spacing is given by s = λD/d, where λ is wavelength, D is the distance from slits to screen, and d is the slit separation. This applies when D is much larger than d — the small-angle approximation the IB always assumes in this topic.

Constructive interference happens when two waves arrive in phase, path difference equal to a whole number of wavelengths (nλ), producing a bright fringe. Destructive interference happens when they arrive out of phase, path difference (n+½)λ, producing a dark fringe. Both come straight from superposition — you're adding displacements, not intensities, at each point.

Basic two-source interference and the double-slit formula appear on both SL and HL papers. Deeper treatment — diffraction gratings, thin-film interference, and multiple-slit patterns — is HL-only content within Topic 9. Check the current subject guide's syllabus content column, since 'HL only' is marked explicitly there.

Expect 'state' (give the coherence condition), 'explain' (why fringes form using path difference), 'calculate' (fringe spacing, wavelength), 'sketch' (intensity distribution) and 'compare' (single-slit diffraction vs double-slit interference patterns). Each command term carries a different mark allocation, so matching your answer's depth to the verb genuinely matters.

The most frequent one I see in mock papers: forgetting to convert slit separation and wavelength into consistent units (mm vs nm vs m) before substituting into s = λD/d. The second most common: mixing up d (slit separation) with s (fringe spacing) in the formula, which flips the whole calculation.

The double-slit pattern is the standard evidence that light behaves as a wave, since only wave superposition explains the fringes. IB Physics also covers the electron/photon version, where particles fired one at a time still build up an interference pattern — direct evidence for wave-particle duality, tested in Topic 12 (Quantum and Nuclear Physics).

Master the formula, then practise the qualitative explanations just as hard — most students lose marks on 'explain' questions, not calculations. I always tell my students: draw the wavefront diagram for path difference, label nλ and (n+½)λ, and you'll rarely misexplain a fringe pattern again.

Try this: a laser of wavelength 589 nm shines through slits 0.30 mm apart, screen 1.5 m away. Find the fringe spacing, then find how far the 5th bright fringe sits from the centre. This mirrors a typical Paper 1B/2 structure — one straightforward calculation, one that extends it.

It's moderately demanding — the maths is straightforward algebra, but the conceptual explanations (coherence, path difference, superposition) trip up students who've only memorised the formula. Students who've done well in GCSE/IGCSE or MYP-level wave physics usually adapt quickly; those newer to wave concepts need a bit more time on the 'why', not just the 'how'.

Interference underpins spectroscopy, laser technology, fibre optics and even gravitational-wave detection (LIGO uses laser interferometry) — it's a genuine foundation topic, not an isolated exam curiosity. Universities offering physics, engineering or optics-related degrees expect this concept to be second nature by first year, so a solid grasp here pays off well beyond the IB exam.

The most efficient combination is concise revision notes for the theory, topical worksheets for formula practice, and full past papers under timed conditions closer to exams. RevisionPrep's Physics resources cover Wave Phenomena with worked examples and a graded question bank, so your child can move from guided practice to exam-style questions without needing separate materials.