IB Physics quantum physics at HL is concentrated in E.2 Quantum physics, an HL-only topic in the syllabus first assessed in 2025. Its examinable core is smaller than many students expect: the photoelectric effect, matter-wave diffraction, the de Broglie wavelength, and Compton scattering. Success depends on connecting each experiment to wave-particle duality, interpreting graphs accurately, and showing calculations in a form that earns method marks.
This guide explains those ideas with an exam lens. After learning the theory, use the E.2 Quantum Physics Questionbank and its per-question worked or video solutions where available to see how the method converts into marks.
What does IB Physics HL require for quantum physics?
Under the current course, E.2 Quantum physics is additional higher level content with approximately eight recommended teaching hours. There is no SL content within E.2, although related atomic and nuclear ideas appear elsewhere in Theme E.
The official syllabus centres on four connected areas:
| Area | Essential idea | Typical exam task |
|---|---|---|
| Photoelectric effect | Light transfers energy in photons | Explain observations, calculate energy, interpret graphs |
| Matter waves | Moving particles have wavelength | Calculate de Broglie wavelength or explain electron diffraction |
| Wave-particle duality | Light and matter display wave-like and particle-like behaviour | Identify what an experiment demonstrates |
| Compton scattering | Photons transfer energy and momentum to electrons | Calculate wavelength shift or explain why wavelength increases |
Older IB resources may include wave functions, uncertainty relations, pair production, and quantum tunnelling under “Topic 12.” These belonged to the pre-2025 course and are not listed in current E.2. Current students should follow the syllabus used by their examination session rather than relying on an old topic number.
The revised assessment has Paper 1A multiple-choice questions, Paper 1B data-based questions, and Paper 2 short- and extended-response questions. Quantum physics can therefore be assessed through rapid calculations, unfamiliar experimental data, graph interpretation, or a structured explanation. The official IB specimen papers show the current paper style.
Photons and the photoelectric effect
A photon is a quantum of electromagnetic radiation with energy and momentum:
$E = hf = hc/λ$$p = h/λ = E/c$
Here, $h$ is Planck’s constant, $f$ is frequency, and $λ$ is wavelength. Higher-frequency radiation therefore consists of more energetic photons, while increasing intensity at a fixed frequency means that more photons arrive per second.
In the photoelectric effect, photons incident on a metal can eject electrons. One photon transfers its energy to one electron. Some energy overcomes the metal’s work function Φ, the minimum energy needed to release an electron, and the remainder becomes kinetic energy:
$E_max = hf - Φ$
Emission occurs only if $hf ≥ Φ$. The corresponding threshold frequency is $f₀ = Φ/h$.
What examiners expect in explanations
A strong explanation distinguishes frequency from intensity:
- Increasing frequency increases the energy of each photon and therefore increases the maximum kinetic energy of emitted electrons.
- Increasing intensity at fixed frequency increases the number of photons arriving each second and therefore usually increases the photocurrent.
- If the frequency is below the threshold, increasing intensity does not cause emission because no individual photon has sufficient energy.
- Emission above the threshold is effectively immediate, supporting one-photon, one-electron energy transfer.
Classical wave theory associates greater intensity with greater delivered energy, so it would suggest that sufficiently intense light should eventually eject electrons at any frequency. It also struggles to explain why electron kinetic energy depends on frequency rather than intensity. The photon model explains both observations.
Photoelectric graphs and stopping potential
Graph questions are particularly efficient sources of marks when each feature is interpreted physically. For a graph of maximum kinetic energy against frequency,
$E_max = hf - Φ$
has the form $y = mx + c$.
| Graph feature | Physical meaning |
|---|---|
| Gradient | Planck’s constant $h$ |
| Frequency-axis intercept | Threshold frequency $f₀$ |
| Energy-axis intercept | $-Φ$ |
If emitted electrons are opposed by a potential difference, the stopping potential $V_s$ is the minimum reverse potential that reduces the photocurrent to zero. The fastest photoelectrons then satisfy:
$eV_s = E_max = hf - Φ$
For example, photons of energy 4.0 eV incident on a metal with work function 2.3 eV produce photoelectrons with maximum kinetic energy 1.7 eV. The stopping potential has a numerical value of 1.7 V because an electron moving through 1.7 V changes energy by 1.7 eV.
A common mistake is to say that every photoelectron has this kinetic energy. Electrons originate at different depths and can lose different amounts of energy before leaving, so $E_max$ describes the maximum value.
Matter waves and the de Broglie wavelength
De Broglie proposed that matter has wave-like properties. A particle with momentum $p$ has wavelength:
$λ = h/p$
For a non-relativistic particle, $p = mv$, giving $λ = h/mv$. Greater mass or speed produces greater momentum and therefore a shorter wavelength.
If a particle of charge magnitude $q$ is accelerated from rest through potential difference $V$, then:
$qV = ½mv² = p²/(2m)$
Combining this with the de Broglie relation gives:
$λ = h/√(2mqV)$
This derived expression is useful in electron-diffraction questions. Check that the particle begins from rest and that the speed is low enough for non-relativistic mechanics before using it.
Why electron diffraction matters
When electrons pass through a thin crystalline material, they produce diffraction patterns. Diffraction is a wave phenomenon, so this is evidence that particles possess wave-like behaviour.
Examiners may ask students to “explain how the observation provides evidence for the wave nature of matter.” Simply stating that electrons diffract is often incomplete. A better response identifies electrons as particles, identifies diffraction or interference as wave behaviour, and concludes that matter exhibits wave-particle duality.
Compton scattering
In Compton scattering, a photon interacts with an electron and emerges in a different direction with a longer wavelength. The photon transfers energy and momentum to the electron, so the scattered photon has less energy, lower frequency, and greater wavelength.
The wavelength shift is:
$Δλ = λ_f - λ_i = [h/(m_e c)](1 - cos θ)$
Here, $θ$ is the photon’s scattering angle. The quantity $h/(m_e c)$ is the electron Compton wavelength, approximately 2.43 × 10⁻¹² m, consistent with current NIST fundamental constants.
Important limiting cases are:
$θ = 0°$:$Δλ = 0$, so there is no wavelength shift.$θ = 90°$:$Δλ = h/(m_e c)$.$θ = 180°$: the shift is maximum,$Δλ = 2h/(m_e c)$.
Compton scattering supports the particle model of light because the result is explained as a collision in which a photon carries momentum and transfers energy and momentum to an electron. Do not describe it merely as “proof that light is a particle.” The precise conclusion is that light displays particle-like behaviour under these experimental conditions.
How IB exam questions phrase quantum physics
Command terms determine the required depth. Watch for these common patterns:
- State: give a concise fact or equation, such as
$λ = h/p$. - Calculate: substitute correctly, retain units, and show enough working for method marks.
- Determine: extract information from data or a graph before calculating.
- Explain: connect cause and effect using photons, energy, momentum, or diffraction.
- Discuss: consider several observations or compare classical and quantum predictions.
- Show that: begin from supplied information and produce the stated result without treating it as an assumption.
Paper 1A may test proportional reasoning, such as recognising that doubling momentum halves de Broglie wavelength. Paper 1B may provide experimental photoelectric or scattering data and ask for a gradient, uncertainty, or physical interpretation. Paper 2 is more likely to combine explanation with a multi-step calculation.
Common mistakes that lose marks
- Confusing photon energy with light intensity.
- Using wavelength directly in
$E = hf$without first converting through$f = c/λ$. - Forgetting that the work function and photon energy must use compatible units.
- Treating
$E_max$as the kinetic energy of every emitted electron. - Using the electron’s deflection angle instead of the photon scattering angle in the Compton equation.
- Claiming that wave-particle duality means an object is literally switching between two physical forms.
- Revising removed material from the old syllabus while neglecting current E.2 graph skills.
Write equations symbolically before substituting and check whether energies are in joules or electronvolts. The RevisionDojo IB Physics data booklet resource can help you practise locating equations rather than relying entirely on memorisation.
An efficient revision method
Start with the E.2 lesson sequence or E.2 study notes. Then complete short sets covering one skill at a time: photoelectric graphs, de Broglie calculations, explanations of diffraction, and Compton shifts.
After each set, classify errors as conceptual, equation selection, unit conversion, graph interpretation, or command-term errors. Use Jojo AI to obtain mark-scheme-aligned feedback, but rewrite the corrected response yourself. Finally, watch the per-question worked video solutions in RevisionDojo’s physics practice resources to see how diagrams, equations, and explanations are organised under exam conditions.
Conclusion
Current IB Physics quantum physics at HL is built around a manageable group of ideas: photons explain the photoelectric effect, de Broglie waves explain particle diffraction, and Compton scattering demonstrates photon momentum. The highest-value preparation combines accurate theory with graph interpretation, clear command-term responses, and repeated calculations.
RevisionDojo’s IB Physics resource hub brings these stages together. Use the E.2 Questionbank for targeted practice, Jojo AI for feedback, and the worked or video solutions to compare your method with an exam-ready approach.
Sources and referenced URLs
- Official IB Diploma Programme physics overview
- Official IB Physics specimen papers for first examinations in 2025
- IB Physics curriculum and assessment updates
- Current Physics guide copy containing E.2 syllabus details
- NIST CODATA fundamental physical constants
- OpenStax explanation of the photoelectric effect
- OpenStax explanation of Compton scattering
- RevisionDojo E.2 Quantum Physics Questionbank
- RevisionDojo E.2 Quantum Physics lessons
- RevisionDojo E.2 Quantum Physics notes
- RevisionDojo IB Physics data booklet resource
- RevisionDojo IB Physics resource hub and videos