A-Level · Physics · AQA · Mark scheme decoded

AQA A-Level Physics: Wave-Particle Duality and Electron Diffraction — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Physics specificationlast verified 3 July 2026

The short answer

The concept of wave-particle duality is a fundamental principle in modern physics, suggesting that particles can exhibit both wave-like and particle-like properties. This idea was revolutionary when it was first proposed and has profound implications for our understanding of the nature of matter and electromagnetic radiation.

The question

An electron has a momentum of 5 × 10 -24 kg m/s. Calculate its de Broglie wavelength.

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For calculation questions, marks are typically awarded for correct substitution of values into the formula, accurate arithmetic, and appropriate units. For explanation and description questions, marks are given for clarity, completeness, and relevance to the topic.

What the command words demand

Calculate: Perform a numerical calculation using given data and appropriate formulas.
Explain: Provide a clear and detailed account of a concept or phenomenon, including relevant scientific principles.
Describe: Give a detailed account of the characteristics or features of a topic without necessarily explaining why they occur.
Verify: Check the consistency of given values with a known equation or principle.
Evaluate: Assess the strengths and weaknesses of a theory or experimental method.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the question, perform calculations, and provide well-structured answers.

Identify the given values and the formula to use.1 mark
Given: Momentum (mv) = 5 × 10 -24 kg m/s, Planck's constant (h) = 6.626 × 10 -34 Js
Substitute the values into the de Broglie wavelength equation λ = h / mv.1 mark
λ = (6.626 × 10 -34 Js) / (5 × 10 -24 kg m/s)
Perform the division to find the wavelength.1 mark
λ = 1.3252 × 10 -10 m
Express the answer in a suitable unit, such as nanometers (nm).1 mark
1.3252 × 10 -10 m = 0.13252 nm

Final answer: 0.13252 nm

Work through every step correctly and you earn all 4 marks.

Another worked example

A photon has a wavelength of 600 nm. Calculate its momentum.

4 marks

Identify the given values and the formula to use.1 mark
Given: Wavelength (λ) = 600 nm = 600 × 10 -9 m, Planck's constant (h) = 6.626 × 10 -34 Js
Rearrange the de Broglie wavelength equation to solve for momentum (mv).1 mark
mv = h / λ
Substitute the values into the rearranged equation.1 mark
mv = (6.626 × 10 -34 Js) / (600 × 10 -9 m)
Perform the division to find the momentum.1 mark
mv = 1.1043 × 10 -27 kg m/s

Final answer: 1.1043 × 10 -27 kg m/s

Work through every step correctly and you earn all 4 marks.

Common mistakes

Confusing the de Broglie equation with the speed of light equation (c = λf).
Why it happens: Students sometimes mix up different equations involving wavelength, especially when dealing with multiple concepts like wave-particle duality.
Fix: Review and memorize the specific form of the de Broglie equation: λ = h / mv. Practice using it in various contexts to reinforce its application.
Forgetting to convert units when necessary, such as converting meters to nanometers or picometers.
Why it happens: Unit conversion is a common oversight, especially when dealing with very small values like wavelengths of particles.
Fix: Always check the units required in the question and perform necessary conversions. Practice problems that involve different units to build familiarity.
Using the wrong value for Planck's constant (h).
Why it happens: Students may use an incorrect or approximate value for Planck's constant, leading to inaccurate calculations.
Fix: Memorize the exact value of Planck's constant: 6.626 × 10 -34 Js. Double-check your work to ensure you are using the correct value.
Misinterpreting the relationship between momentum and wavelength in the de Broglie equation.
Why it happens: Students may not fully understand that increasing momentum decreases wavelength, and vice versa.
Fix: Review the inverse relationship between momentum (mv) and wavelength (λ) as described by λ = h / mv. Practice problems that involve changing momentum to see how it affects the wavelength.
Failing to explain the significance of electron diffraction in supporting wave-particle duality.
Why it happens: Students may focus on the mathematical aspects and overlook the conceptual importance of experimental evidence like electron diffraction.
Fix: Understand that electron diffraction provides strong evidence for the wave-like properties of particles. Practice explaining this concept in both written and verbal contexts.
Not recognizing the role of peer review in scientific research.
Why it happens: Students may not fully appreciate the importance of peer review in validating new theories and ensuring scientific rigor.
Fix: Study the process of peer review and its significance in advancing scientific knowledge. Practice explaining how peer review contributes to the credibility of new discoveries.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Calculate De Broglie Wavelength	Use λ = h/mv to find an electron's de Broglie wavelength from its momentum.	4
Calculate Photon Momentum	Use the de Broglie relation to find a photon's momentum from its wavelength.	4
Calculate Momentum	Rearrange the de Broglie equation to find momentum from a given wavelength.	4
Verify De Broglie Equation	Calculate the wavelength from momentum and compare it with the given value for consistency.	4
Total across these question types		16

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.