AQA AS Level Physics Paper 1, June 2022: Question 5
5 marks · Medium difficulty · Extended Answer
Explain wave-like behavior of electrons at a graphite target and how increasing electron speed affects the diameter of diffraction rings.
Practise this questionQuestion
Question text
05 Figure 4 shows apparatus used to demonstrate the wave–particle duality of electrons.
Figure 4
The heated filament emits slow-moving electrons.
In region P, the electrons are accelerated to a high speed.
At Q, the fast-moving electrons are incident on the graphite target.
R is a point on one of the bright rings that are formed where the electrons strike the
fluorescent screen.
05.1 The electrons demonstrate wave-like and particle-like behaviour as they travel from
the filament to the screen.
State and explain at which of P, Q or R the electrons are demonstrating wave-like
behaviour.
[2 marks]
05.2 The apparatus is adjusted so that the electrons are incident on the graphite target with
a greater speed.
Explain why the bright rings formed on the screen now have a smaller diameter.
[3 marks]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidance Mark AO
05.1 Q Talk out of Q where diffraction linked to any 2 2 x AO3
other location (positive plate or screen)
Accept gaps between (graphite) atoms acts as
Diffraction as the electron move between the layers in the graphite/
slits for electrons to diffract through
electrons spread out as they move between the layers in the
graphite Graphite acts like a diffraction grating is not
enough.
Talk out where particle property is used to
describe interaction between the electrons and
the graphite (e.g. electrons repelled by
graphite)
Treat interference at R as neutral.
Treat interference at Q as neutral
Allow maximum of one mark for describing
particle behaviour at P or R with a reason
given:
- acceleration is a particle phenomenon (P)
- fluorescence is due to a collision with atomic
electron which is particle phenomenon. (R)
05.2 Treat double slit formula as neutral 3 3 x AO2
decreases (associated) wavelength / Momentum of electrons 17
increases
MP1 and MP2: talk out on use of wave
equation / talk out on frequency remaining
constant / talk out on frequency increases
h
quotes λ = / wavelength is inversely proportional to speed
mv
/ wavelength is inversely proportional to momentum
less diffraction because shorter wavelength relative to the
Accept: less diffraction because shorter
spacing between layers in the graphite / less diffraction
wavelength relative to size of slits
because shorter wavelength relative to gaps (in graphite
target)
Where no other mark is scored allow 1 mark
for:
less diffraction
‘Spreads out less’ is insufficient here
Total 5
How to answer it
Electron Diffraction and Wave-Particle Duality Study Guide
This question assesses your understanding of de Broglie wavelength, electron diffraction through atomic spacings (graphite crystals), and how altering particle momentum impacts wave behavior. You must link mathematical relationships (like the de Broglie equation) to physical observations on a fluorescent screen.
Question 05.1: Identifying Wave-Like Behaviour
State and explain at which of P, Q or R electrons demonstrate wave-like behaviour. [2 marks]
✅ Correct Answer
- Location: Q
- Explanation: Diffraction occurs as the electrons pass through/between the layers of the graphite target (acting like a diffraction grating).
💡 Key Knowledge
- Diffraction is a strictly wave-like phenomenon.
- The spacing between atoms/layers in graphite is on a similar scale to the de Broglie wavelength of accelerated electrons (approx. 10⁻¹⁰ m).
- Regions P and R demonstrate particle characteristics (acceleration by an electric field and particle collision causing fluorescence respectively).
🧠 Exam Technique
- State the letter clearly first to secure the first mark.
- Explicitly link the phenomenon ("diffraction") to the structure ("graphite layers/gaps") to unlock the second mark.
❌ Common Errors
- Choosing P or R and trying to justify wave behavior (acceleration at P and impact at R are particle interactions).
- Stating "graphite acts like a diffraction grating" without mentioning the physical mechanism (electrons passing between layers/atoms).
- Confusing diffraction with interference.
Question 05.2: Effect of Greater Speed on Ring Diameter
Explain why the bright rings formed on the screen now have a smaller diameter when electrons have greater speed. [3 marks]
✅ Correct Answer
- Greater speed leads to increased momentum and a smaller de Broglie wavelength ( λ = h / mv ).
- A smaller wavelength results in less diffraction ( sin θ is smaller because λ is smaller relative to slit/atomic spacing).
- Less diffraction means the rings are deflected by smaller angles, forming closer to the center, hence a smaller diameter.
💡 Key Knowledge
- De Broglie equation: λ = h / p or λ = h / mv .
- Wave property relationship: As wavelength decreases, the amount of observable diffraction decreases.
🧠 Exam Technique
- Structure your answer logically in 3 clear stages: Velocity/Momentum change → Wavelength change → Degree of diffraction change → Ring size change.
- Avoid vague phrases like "it spreads out less" without first explaining the wavelength change.
📐 Step-by-Step Logic
- Step 1: Speed ( v ) increases &rِّفarr; momentum ( p = mv ) increases.
- Step 2: Quote λ = h / mv to show wavelength ( λ ) decreases.
- Step 3: Relate smaller λ to graphite atom spacing to conclude there is less diffraction, giving a smaller ring diameter.
Mark 2: Quotes λ = h / mv or states inverse proportionality.
Mark 3: Links smaller wavelength to less diffraction / smaller angle deviation.
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA AS Level Physics examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.