IB Physics Wave–Particle Duality FAQ

You need three things: the photoelectric effect as evidence light behaves as particles (photons), the de Broglie hypothesis that matter has a wavelength, and the ability to explain why classical wave or particle models alone fail. According to the IB Physics guide (first exams 2025), this sits under Theme E: Nuclear and quantum physics, sub-topic E2.

The photoelectric effect is when light striking a metal surface ejects electrons instantly, but only above a threshold frequency — regardless of intensity. Wave theory predicts brighter light should always eject electrons eventually; it doesn't. Einstein explained this by treating light as discrete photons, each carrying energy $E=hf$, which won him the 1921 Nobel Prize.

De Broglie proposed that every moving particle has an associated wavelength given by $\lambda =h/p$, where $h$ is Planck's constant and $p$ is momentum. This means electrons, protons, even footballs, have a wave nature — though it's only detectable at atomic scales, which is why you never notice a thrown ball diffracting.

Wave-particle duality is core content for both SL and HL — the photoelectric effect and de Broglie wavelength appear on every student's paper. HL students get one extra layer: the Heisenberg uncertainty principle, which only appears in the HL extension of the same sub-topic.

Firing single electrons through a double slit, one at a time, still builds up an interference pattern over time — proving each electron interferes with itself as a wave, yet lands as a single point like a particle. This is the classic demonstration examiners reference when asking you to justify duality with evidence.

Only HL students are examined on it, as part of the Theme E quantum physics extension. It states you can't simultaneously know a particle's exact position and momentum — the more precisely you pin down one, the less precisely you know the other, expressed as $\Delta x\Delta p\ge h/4\pi$.

Expect it in both. Paper 1 (multiple choice) often tests quick recall — photon energy calculations, threshold frequency reasoning. Paper 2 (short/extended response) is where duality really bites, asking you to explain evidence and compare models in 3-6 mark written answers.

The biggest one: confusing photon energy ($E=hf$) with photon momentum ($p=E/c$ or $p=h/\lambda$) and using the wrong equation in calculations. The second: writing vague answers like "light acts weird" instead of naming the specific experiment and model it contradicts.

It's conceptually hard, not mathematically hard — the equations are short, but the ideas contradict everyday intuition, and IB examiners specifically reward students who can reason through why classical physics fails rather than just quoting a formula. That's the skill most revision guides skip.

Photon energy is $E=hf$ (depends on frequency alone). Photon momentum is $p=E/c=h/\lambda$ (depends on wavelength). Students often plug frequency into the momentum formula or vice versa — always check which variable the question gives you before choosing the equation.

Past-paper questions on this topic tend to repeat the same few command terms — "explain," "outline," "suggest" — so past-paper practice matters more here than in most quantum topics. Revision Notes with worked examples, topical worksheets isolating quantum physics, and full Mock Papers under timed conditions all help your child build the exact answer structure examiners reward.