AQA AS Level Physics Paper 1, June 2018: Question 7
12 marks · Hard difficulty · Extended Answer
Describe the interference patterns formed using white light, green and red light filters, and discuss the effect of changing slit separation and slit-screen distance on uncertainty.
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Question text
07 Figure 8 shows a diagram of apparatus used to demonstrate the formation of
interference fringes using a white light source in a darkened room. Light from the
source passes through a single slit and then through two narrow slits S1 and S2.
Figure 8
07.1 Describe the interference pattern that is seen on the white screen.
[2 marks]
07.2 A filter transmits only green light of wavelength and red light of wavelength 1.2
This filter is placed between the light source and the single slit.
Describe the interference pattern now seen on the white screen.
Use a calculation to support your answer.
[4 marks]
07.3 A student decides to use the apparatus shown in Figure 8 to determine the
wavelength of red light using a filter that transmits only red light.
The student suggests the following changes:
*23* • decrease slit separation s
• decrease D, the distance between the slits and the screen.
The student decides to make each change independently.
Discuss the effects each independent change has on the interference pattern, and
whether this change is likely to reduce uncertainty in the determination of the
wavelength.
[6 marks]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidelines Mark
TWO FROM:
central white fringe
(fringes either side) showing range of colours/spectrum Allow rainbow for spectrum
07.1 1
with red furthest and blue/violet closest to centre Reject different colour fringes
(MAX 2)
If colours mentioned for last mark must be in right
order i.e. red last
FOUR FROM:
central fringe is a mixture of red and green light/two wavelengths allow orange/yellow for central fringe
EITHER (1 marks)
(separate) red and green fringes are seen (on either side)
OR (for 2 marks) 1
07.2 spacing of green fringes is less than spacing of red fringe / green if w used must be identified as fringe spacing for 1
fringes closer to middle than red third alternative 1
(MAX 4)
OR (for 3 marks)
spacing of red fringes is 20% (or 1.2 times)greater than green
fringes
6th green fringe overlaps with 5th red fringe
– – –
The mark scheme gives some guidance as to what statements are The following statements may be present for
expected to be seen in a 1 or 2 mark (L1), 3 or 4 mark (L2) and 5 or 6 decreasing slit separation s:
mark (L3) answer. Guidance provided in section 3.10 of the ‘Mark
Scheme Instructions’ document should be used to assist in marking Fringe separation increases
this question.
Uncertainty in measuring fringe separation will
Mark Criteria QoWC decrease
6 Explains how (%) The student presents and as this is needed to measure wavelength,
uncertainties combine to relevant information uncertainty in wavelength
determine uncertainty in coherently, employing 15
measurement will decrease
wavelength OR identify % structure, style and sp&g
uncertainty s as being the to render meaning clear. The following statements may be present for
largest The text is legible. smaller D:
5 Explain how wavelength is
𝑤𝑠
determined using = 𝐷 Uncertainty in measuring D would increase
Fringe separation would also decrease
so uncertainty in measuring fringe separation would
increase
Both are required to find wavelength so uncertainty
07.3 6
in finding wavelength would increase
4 Explains how second change The student presents FOR Middle Band one of these considered:
affects fringe spacing relevant information and in
a way which assists Decrease s
AND communication of Larger fringe separation so smaller (%) uncertainty
meaning. The text is (in w)
Comments on how change legible. Sp&g are Smaller s so higher (%) uncertainty in s
in fringe spacing affects sufficiently accurate not to Decrease D
(%)uncertainty / change in s obscure meaning. Smaller fringe separation so larger (%) uncertainty
OR D affects (%)uncertainty (in w)
Smaller D so higher (%) uncertainty in D
How to answer it
Interference Fringes and Young's Double Slit Experiment
What this question tests
This question assesses your understanding of wave optics, specifically superposition, interference patterns using continuous spectra (white light) and multi-wavelength sources, fringe spacing calculations using w = λD / s , and the evaluation of experimental uncertainties when modifying laboratory setups.
White Light Interference Pattern
Describe the interference pattern that is seen on the white screen. (2 marks)
✅ Correct Answer
- Central white fringe.
- Fringes either side showing a range of colours / continuous spectrum (or rainbow).
- Red furthest from the centre and blue/violet closest to the centre.
💡 Key Knowledge
White light contains all visible wavelengths. The path difference for the central maximum is zero for all wavelengths, meaning they reinforce constructively at the same central position to form white light. For higher orders, fringe spacing depends on wavelength ( w ∝ λ ), causing the spectra to spread out.
❌ Common Errors
Students often lose marks by stating there are "different coloured fringes" without mentioning the central white fringe or incorrectly reversing the spectrum order (placing blue on the outside and red on the inside).
Two-Wavelength Interference Pattern & Calculation
A filter transmits only green light of wavelength λ and red light of 1.2λ. Describe the interference pattern and use a calculation to support your answer. (4 marks)
✅ Correct Answer
- Central fringe is a mixture of red and green light (appears yellow/orange).
- Separate green and red fringes are seen on either side.
- Green fringes are closer together than red fringes (spacing of red is 20% or 1.2 times greater than green).
- The 6th green fringe overlaps precisely with the 5th red fringe ( 6 × λ = 5 × 1.2λ = 6λ ).
📐 Calculation Support
- Fringe Spacing Ratio: Since w = λD / s , the fringe spacing is directly proportional to wavelength ( w ∝ λ ).
- Scale Factor: Red wavelength is 1.2λ , so red fringe spacing is 1.2 times larger than green.
- Overlap Condition: Find where n_green × λ_green = n_red × λ_red .
n_green × λ = n_red × 1.2λ → n_green = 1.2 × n_red . Testing integers gives 6 × λ = 5 × 1.2λ , proving the 6th green fringe overlaps the 5th red fringe.
🧠 Exam Technique
To secure all 4 marks, combine a clear qualitative description of the overlapping colours with a mathematical justification demonstrating the proportional relationship between the wavelengths and fringe positions.
Evaluating Experimental Changes and Uncertainty
Discuss the effects of independently decreasing slit separation (s) and decreasing slit-to-screen distance (D) on the interference pattern and experimental uncertainty in determining wavelength. (6 marks)
💡 Key Knowledge: Equation Linking
The working formula is λ = ws / D . To find percentage uncertainty in λ , add the percentage uncertainties of each measured parameter:
%Δλ = %Δw + %Δs + %ΔD .
✅ Detailed Discussion Breakdown
- Effect 1: Decreasing slit separation ( s )
Fringe separation ( w ) increases ( w ∝ 1/s ). A larger fringe separation reduces the percentage uncertainty when measuring w across multiple fringes. However, making s too small increases the percentage uncertainty in measuring s itself. - Effect 2: Decreasing distance ( D )
Fringe separation ( w ) decreases ( w ∝ D ). Smaller fringes make it harder to measure distance accurately, increasing the percentage uncertainty in w .
🧠 Examiner Strategy (Level of Response)
Top-level (Level 3) responses explicitly link how percentage uncertainties combine ( %Δλ = %Δw + %Δs + %ΔD ) and weigh up the trade-off: decreasing s improves w measurement precision but degrades s measurement precision.
Topics
Physics · Practical skills · 3.3 Waves · Uncertainty and evaluation
Question and mark scheme from the AQA AS Level Physics examination, Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.