AQA AS Level Physics Paper 2, June 2022: Question 20

1 mark · Medium difficulty · Multiple Choice

Identify which combination of core and cladding refractive indices produces the smallest modal dispersion in an optical fibre.

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Question

Multiple choice question asking which combination produces the smallest modal dispersion in an optical fibre. A table lists four options (A, B, C, D) with refractive index values for the core and cladding respectively: A has core 1.5 and cladding 1.4; B has core 1.4 and cladding 1.5; C has core 1.5 and cladding 1.3; D has core 1.3 and cladding 1.5.
Question text

20 Which combination produces the smallest modal dispersion in an optical fibre?

[1 mark]

Refractive index of core Refractive index of cladding

A 1.5 1.4

B 1.4 1.5

C 1.5 1.3

D 1.3 1.5

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer is option A, corresponding to a core refractive index of 1.5 and a cladding refractive index of 1.4.

20 A (AO2) 1.5 1.4

How to answer it

Optical Fibre Modal Dispersion

What this question tests

This question assesses your understanding of optical communication physics, specifically how the refractive indices of the core and cladding influence modal dispersion in step-index optical fibres. You must understand the physical requirement for total internal reflection and how minimising the difference between core and cladding refractive indices reduces multipath pulse broadening.

Question Part 20 [1 Mark]

Determining the Smallest Modal Dispersion

✅ Correct Answer

Option A (Core refractive index = 1.5 , Cladding refractive index = 1.4 )

💡 Key Knowledge

  • Total Internal Reflection (TIR): The core refractive index ( n₁ ) must always be greater than the cladding refractive index ( n₂ ) for light to trap and travel down the fibre. This immediately rules out options B and D.
  • Cause of Modal Dispersion: Light rays take different paths (modes) down the core. Rays entering at larger angles travel a longer total distance than paraxial (near-axial) rays, causing the light pulse to spread out (broaden).
  • Minimising Dispersion: A smaller difference between n₁ and n₂ restricts the maximum possible angle of acceptance, meaning all permitted rays travel paths very close to the axis, reducing the time difference between modes.

🧠 Exam Technique

  • Rule Out Violations First: Check n₁ > n₂ . Options B and D have core indices smaller than cladding indices—these will refract light out of the core entirely, so eliminate them straight away.
  • Compare Remaining Magnitudes: Between options A and C, look at the delta (difference) between core and cladding:
    • Option A difference: 1.5 - 1.4 = 0.1
    • Option C difference: 1.5 - 1.3 = 0.2
  • Select the smaller difference to minimise dispersion.

❌ Common Errors

  • Confusing Dispersion Types: Some students confuse modal dispersion with material dispersion and mistakenly look for high refractive index values overall.
  • Reversing Core/Cladding Roles: Forgetting which component requires the higher refractive index, leading candidates to select option B.
  • Maximising vs Minimising: Misreading the question stem and choosing option C because a larger numerical difference stands out more.
Examiner Note: This is an AO2 application question. Full marks require candidates to recognise that a small refractive index difference between core and cladding leads to a small range of possible ray angles, thereby reducing modal dispersion while maintaining total internal reflection conditions.

Topics

Physics · 3.3 Waves

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.