What is wave-particle duality in quantum physics?

Wave-particle duality is a fundamental concept in quantum physics that describes how every particle or quantum entity, such as electrons and photons, exhibits both wave-like and particle-like properties. This duality is central to understanding phenomena at the quantum level and is a key topic in the Cambridge International A Levels Physics curriculum.

How does the double-slit experiment demonstrate wave-particle duality?

The double-slit experiment demonstrates wave-particle duality by showing that particles like electrons create an interference pattern, a characteristic of waves, when not observed. However, when observed, they behave like particles, hitting the screen in distinct spots. This experiment highlights the dual nature of quantum entities and is a classic demonstration in quantum physics studies.

Why is wave-particle duality important in quantum physics?

Wave-particle duality is crucial in quantum physics because it challenges classical concepts of particles and waves, leading to a deeper understanding of the nature of matter and energy. It has significant implications for the development of technologies such as quantum computing and is a fundamental concept covered in the Cambridge International A Levels Physics syllabus.

Can you give an example of wave-particle duality in everyday life?

An everyday example of wave-particle duality is light. Light behaves as a wave when it diffracts and interferes, as seen in phenomena like rainbows and the bending of light around objects. However, it also behaves as a particle, as demonstrated by the photoelectric effect, where light knocks electrons off a metal surface. This duality is a key topic in quantum physics education.

How does wave-particle duality relate to the uncertainty principle?

Wave-particle duality is closely related to the Heisenberg Uncertainty Principle, which states that certain pairs of physical properties, like position and momentum, cannot be simultaneously known to arbitrary precision. The wave-like nature of particles leads to inherent uncertainties in their properties, reflecting the probabilistic nature of quantum physics, a concept explored in the Cambridge International A Levels Physics curriculum.

Why is "Looking for de Broglie wavelength of macroscopic objects" a common mistake in Wave-particle duality?

Why it happens: Curiosity. How to avoid it: For massive objects, is far below any observable scale ( 10^-34 m). Wave behaviour only matters for atoms, electrons, photons. Source: 9702 Examiner Reports 2022-2024

Quantum PhysicsCambridge International A Level Physics 9702

Wave-particle duality

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Wave-Particle Duality Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Light + matter exhibit wave AND particle behaviour. de Broglie wavelength.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

22.3.1 — State wave-particle duality.
22.3.2 — Compute de Broglie wavelength.

Wave-particle duality

Two natures.

Light. Behaves as PARTICLE (photoelectric, photon) AND WAVE (diffraction, interference).

Matter (electrons, atoms). de Broglie: also has WAVE property. $\lambda = \frac{h}{p}$

Evidence. Electron diffraction through crystal — distinct pattern, like X-ray diffraction. Confirms electron has wave property.

Scale. $\lambda$ inversely proportional to momentum. Subatomic: detectable. Macroscopic: negligible.

Example. Electron at $5 \times 10^6$ m/s: $\lambda \sim 10^{-10}$ m (atomic scale).

Cambridge tip. Mark scheme rewards naming evidence: 'electron diffraction'.

Electrons through a thin crystal form a diffraction pattern, evidence that matter has a wave nature with de Broglie wavelength λ = h/p.

$\lambda = h/p$ matter.
Both light + matter dual.
Electron diffraction evidence.

See the full worked example for wave-particle duality →

How it’s examined

Duality on Paper 4 typically 5-7 marks.

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/42 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

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Step-by-step worked examples — Wave-particle duality

Step-by-step solutions to past-paper-style questions on wave-particle duality, written exactly the way a tutor would explain them at the board.

1de Broglie wavelength (6 marks)
Extended• Adapted from 9702/42 May/Jun 2024• de Broglie
Question
Find de Broglie wavelength of an electron at $5.0 \times 10^6$ m/s. $m_e = 9.1 \times 10^{-31}$ kg, $h = 6.63 \times 10^{-34}$ . (6 marks)
Step-by-step solution
1. Step 1
  $\lambda = h/p = h/(mv)$ .
  $\lambda = \dfrac{6.63 \times 10^{-34}}{9.1 \times 10^{-31} \times 5 \times 10^6} \approx 1.46 \times 10^{-10} \text{ m}$
Answer
$\lambda \approx 1.46 \times 10^{-10}$ m (similar to atomic spacing).

Key Formulae — Wave-particle duality

The formulae you need to memorise for wave-particle duality on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

de Broglie wavelength
$\lambda = \dfrac{h}{p}$
$p = mv$
momentum of particle
When to use
Wavelength associated with a moving particle.

Key Definitions and Keywords — Wave-particle duality

Definitions to memorise and the exact keywords mark schemes credit for wave-particle duality answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Wave-particle duality
Examiner keyword
Light and matter exhibit BOTH wave-like and particle-like behaviour, depending on experiment.
de Broglie wavelength
Examiner keyword
Wavelength associated with a particle: $\lambda = h/p$ .

Common Mistakes and Misconceptions — Wave-particle duality

The traps other students keep falling into on wave-particle duality questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Looking for de Broglie wavelength of macroscopic objects
9702 Examiner Reports 2022-2024
Why it happens
Curiosity.
How to avoid it
For massive objects, $\lambda$ is far below any observable scale ( $\sim 10^{-34}$ m). Wave behaviour only matters for atoms, electrons, photons.

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Wave-particle duality — frequently asked questions

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Wave-particle duality

Short Study Notes in the form of Slides

Short Study Notes — Wave-particle duality

Detailed Notes

What you’ll learn

How it’s examined

1de Broglie wavelength (6 marks)

de Broglie wavelength

Wave-particle duality

de Broglie wavelength

✕Looking for de Broglie wavelength of macroscopic objects

Practice questions

Past paper style quiz

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Wave-particle duality — frequently asked questions