AQA A-Level Physics Paper 1, June 2024: Question 3
10 marks · Hard difficulty · Extended Answer
Analyze refraction through two joined prisms, the non-astronomical applications of a diffraction grating, and calculate the line spacing and evaluate diffraction options for a given light spectrum.
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Question text
03 Figure 1 shows two prisms A and B of different refractive indices joined to make
a block.
A ray of monochromatic light is shown entering and then leaving the block.
Figure 1
03.1 Complete, on Figure 1, the path of the ray of light inside the block.
[1 mark]
03.2 Deduce which prism, A or B, has the greater refractive index.
[2 marks]
The block is used with a telescope to investigate stars.
The block can be replaced with a diffraction grating.
03.3 Describe one non-astronomical application of a diffraction grating.
[1 mark]
03.4 Figure 2 shows a spectrum of light. Two lines in the spectrum are labelled X and Y.
Figure 2
The light passes at normal incidence through a diffraction grating. The number of
lines per metre for the grating is G.
The first-order diffraction angle of X is at 28.2° to the normal.
Calculate G.
[3 marks]
7 G = m−1
03.5 A scientist wants to obtain an accurate value for the difference in wavelength between
line X and line Y.
She has two options:
• option 1: to analyse the second-order spectrum from the original grating
• option 2: to analyse the first-order spectrum from a grating with 2G lines per metre.
Discuss which option she should choose.
[3 marks]
Mark scheme
Show the mark scheme
Question Answers Additional comments/Guidelines Mark AO
03.1 ray through A links to ray in B Ignore any arrow directions. 1 AO2
AND
ray in B horizontal by eye
03.2 Answer must be consistent with their Figure 2 AO3
1.
Conclusion consistent with their Figure 1
For a correct diagram expect to see B has
greater refractive index / A has lower
refractive index’
10 Supported by consideration of their angles of incidence and
refraction at AB boundary
For a correct diagram expect to see ‘at AB
boundary angle of incidence > angle of
refraction’ OR ‘at AB boundary the ray bends
towards the normal’.
03.3 how the grating is used must be described 1 AO1
e.g ‘used to determine λ of named light
source’ or ‘used to identify elements in a
sample’
Examples:
To analyse chemical composition (of a
sample)
To stabilise/filter laser light
To provide a monochromatic source/select a
particular wavelength of light
appropriate application described In optical encoders for high-precision motor 11
control
Spread evenly the light from e-readers
Condone:
to identify (some) authentic bank notes
applications asociated with entertainment eg
light shows/diffraction glasses.
application associated with analysis of the
light from the Sun
03.4 Ignore POT error in MP1 & MP2. 3 2 × AO2
1 × AO3
MAX 2 from Accept answer in range 377.5 – 382.5 nm.
• λ read from spectrum = 380 nm
• use of d sin θ=n × (their λ ) ‘Use of’ means clear substitution of n, and
θ
1 their λ or rearrangement of equation to give
• use of G = 𝑛𝑛𝑛𝑛
d 𝑑𝑑 = .
sin 𝜃𝜃
If n not seen, assume that n = 1.
Expect to see 8.04 × 10–7 m for d
to give 1.2 × 106 (m–1)
MP2 and MP3 may be seen together
Calculator value range: 1.251790×106 m-1 to
1.2435547×106 m-1 to 1.235427×106 m-1
03.5 3 3 × AO3
Argument involving sinθ = nGλ or equivalent
comparing effect of n = 2 and G’ = 2G
Appreciation that angular separations would be the same
for both options For MP3, allow maxima are better defined in
option 2
Discussion suggesting option 2 / 2G should be used, as
n = 2 spectrum could overlap with other orders obscuring
absorption lines
Alternative for MP3: idea that 2G should be
used as the second-order spectrum would be
dimmer – allow reverse argument that 2G first
order would be brighter.
Total 10
How to answer it
Optics, Refraction & Diffraction Gratings Study Guide
What this question tests
This exam question tests your core understanding of wave optics, specifically refraction across boundaries, Snell's law principles, practical applications of diffraction gratings, multi-step grating calculations (involving lines per metre G and slit spacing d), and analytical evaluation of experimental options.
Question 03.1: Ray Tracing Through Prisms
Complete the path of the ray of light inside the block. [1 mark]
✅ Correct Answer
The ray passing through prism A must link smoothly to the ray entering prism B at the internal boundary, and exit horizontally to the right across the vertical face of prism B.
💡 Key Knowledge
- A normal line is perpendicular to the boundary surface.
- Light entering a medium with a higher refractive index bends towards the normal.
Question 03.2: Refractive Index Deduction
Deduce which prism, A or B, has the greater refractive index. [2 marks]
✅ Correct Answer
Prism B has the greater refractive index (or prism A has the lower refractive index).
🧠 Exam Technique
To secure both marks, your written deduction must directly match your optical drawing in 03.1. Explicitly state that at the AB boundary, the angle of incidence is greater than the angle of refraction, demonstrating that the ray bends towards the normal upon entering prism B.
Question 03.3: Applications of Diffraction Gratings
Describe one non-astronomical application of a diffraction grating. [1 mark]
✅ Correct Answer
Used to determine the wavelength of a named light source, or used to analyse the chemical composition of a sample by observing its atomic line spectra.
❌ Common Errors
Vague statements like "to look at light" or general mentions of "lasers" without explaining how the grating is utilised will fail to score. You must link the grating to its function (e.g., dispersing light into constituent wavelengths).
Question 03.4: Calculating Lines per Metre (G)
The first-order diffraction angle of X is at 28.2 degrees to the normal. Calculate G. [3 marks]
📐 Step-by-Step Calculation
- Read wavelength from graph (Line X): Looking at Figure 2, peak X occurs at 380 nm (Acceptable range: 377.5 nm to 382.5 nm). Convert to metres: 380 × 10⁻⁹ m .
- Recall grating equation: d sin θ = n λ . Here, order n = 1 and angle θ = 28.2° .
- Rearrange for slit spacing (d): d = (n λ) / sin θ = (1 × 380 × 10⁻⁹) / sin(28.2°) = 8.04 × 10⁻⁷ m .
- Calculate lines per metre (G): G = 1 / d = 1 / (8.04 × 10⁻⁷) = 1.24 × 10⁻⁶ m⁻¹ (Acceptable range from mark scheme: 1.24 × 10⁻⁶ m⁻¹ to 1.25 × 10⁻⁶ m⁻¹ ).
❌ Common Calculation Traps
Powers of 10: Forgetting to convert nanometres ( nm ) to metres ( m ) using ×10⁻⁹ is the most frequent source of lost marks.
Question 03.5: Evaluating Experimental Options
Discuss which option she should choose to obtain an accurate value for the difference in wavelength between line X and line Y. [3 marks]
✅ Correct Answer
She should choose option 2 (analysing the first-order spectrum with a grating of 2G lines per metre).
💡 Key Knowledge & Argumentation
- Comparing effects: Using n = 2 (option 1) vs doubling the grating density 2G (option 2) yields equivalent angular separations for the diffraction pattern based on d sin θ = n λ .
- Order overlap mitigation: Option 2 uses the first-order spectrum ( n = 1 ), avoiding spectral lines from higher orders overlapping with each other, which would obscure absorption or emission lines. Alternatively, second-order spectra can suffer from reduced brightness/dimmer images.
Topics
Physics · Optional topics · 3.3 Waves · 3.9 Astrophysics (A-level only)
Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.