AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Mv Students should be able to explain how and why the amount of…
Full specification wording
Wave-particle duality: mv Students should be able to explain how and why the amount of diffraction changes when the momentum of the particle is changed.
This statement asks you to link momentum to the extent of diffraction.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 4
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This statement asks you to link momentum to the extent of diffraction. A particle of momentum p has de Broglie wavelength λ = h/p. Diffraction is most noticeable when λ is comparable to the size of the gap or obstacle. If momentum increases, λ decreases, so the wave is small compared with the gap and spreads less: the pattern narrows and the fringes move closer together. If momentum decreases, λ increases, so the wave is larger relative to the gap and spreads more: the pattern widens and fringes move further apart. To explain this, state the relationship, identify the change in λ, then link that change to the observed spreading.
For example: Doubling the momentum halves λ, which narrows the central maximum.
What earns the marks
- Quote or apply λ = h/p to show that momentum and de Broglie wavelength are inversely proportional.
- State that the amount of diffraction depends on the ratio of wavelength to gap or obstacle size.
- Explain that increasing momentum decreases λ, so the wave spreads less and the diffraction pattern becomes narrower with fringes closer together.
- Explain that decreasing momentum increases λ, so the wave spreads more and the diffraction pattern becomes wider with fringes further apart.
- Use a concrete comparison, such as doubling momentum halving λ and therefore narrowing the pattern, to support the explanation.
Common mistakes and how to put them right
Saying that increasing momentum increases diffraction.
Put it right: The correct link is that higher momentum means shorter wavelength and less spreading.
Forgetting to mention the gap or obstacle size.
Put it right: The amount of diffraction depends on the comparison between λ and that size, not on λ alone.
Mixing up cause and effect by claiming the pattern changes the momentum.
Put it right: The momentum change causes the wavelength change, which causes the pattern change.
Exam tips
- Structure your answer as: momentum change → wavelength change → change in spreading → change in pattern.
- Use comparative language such as 'narrower', 'wider', 'closer together' and 'further apart' to make the direction of change clear.
- If values are given, calculate λ before and after the momentum change to justify your explanation numerically.
Try the lesson
The start of the Wave-particle duality: mv Students should be able to explain how and why the amount of diffraction changes when the momentum of the particle is changed. lesson, made from checked teaching notes for this specification point.
Have a go first
How does the de Broglie wavelength of an electron change if its momentum is doubled?
Listen
This statement asks you to link momentum to the extent of diffraction. A particle of momentum p has de Broglie wavelength λ = h/p. Diffraction is most noticeable when λ is comparable to the size of the gap or obstacle. If momentum increases, λ decreases, so the wave is small compared with the gap and spreads less: the pattern narrows and the fringes move closer together. If momentum decreases, λ increases, so the wave is larger relative to the gap and spreads more: the pattern widens and fringes move further apart. To explain this, state the relationship, identify the change in λ, then link that change to the observed spreading. For example, doubling the momentum halves λ, which narrows the central maximum. A strong answer earns marks for these points. Quote or apply λ = h/p to show that momentum and de Broglie wavelength are inversely proportional. State that the amount of diffraction depends on the ratio of wavelength to gap or obstacle size. Explain that increasing momentum decreases λ, so the wave spreads less and the diffraction pattern becomes narrower with fringes closer together. Explain that decreasing momentum increases λ, so the wave spreads more and the diffraction pattern becomes wider with fringes further apart. Exam tip: Structure your answer as: momentum change → wavelength change → change in spreading → change in pattern.
See it
Key words
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de Broglie wavelength
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Momentum and diffraction
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See every definition
- de Broglie wavelength
- A particle of momentum p has this wavelength given by λ = h/p, so momentum and wavelength are inversely proportional.
- Momentum and diffraction
- Increasing momentum decreases λ, so the wave spreads less and the pattern becomes narrower with fringes closer together; decreasing momentum does the reverse.
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