AQA AS Level Physics Paper 2, June 2025: Question 14

1 mark · Easy difficulty · Multiple Choice

Calculate the wavelength of monochromatic light in glass given its wavelength in air and the refractive index of glass.

Practise this question

Question

Question 14 asks: Monochromatic light passes from air to glass. The light has a wavelength of 632 nm in air. The refractive index of the glass is 1.50. What is the wavelength of the light in the glass? Four options are provided: A 316 nm, B 421 nm, C 632 nm, D 948 nm.

Mark scheme

Show the mark scheme Mark scheme for question 14 indicates that the correct answer is B (421 nm), awarded 1 mark with assessment objective AO1.

How to answer it

Wavelength of Light in an Optically Dense Medium

📌 What this question tests

This question assesses your understanding of refraction at a boundary, specifically how the wave speed and wavelength change while frequency remains constant when light enters a medium with a higher refractive index ( n ).

Question 14

Multiple Choice: Refraction & Wavelength Change [1 Mark]

✅ Correct Answer

B — 421 nm

When light travels into a denser medium (glass, where n = 1.50 ), its speed and wavelength decrease by a factor equal to the refractive index:
λ_glass = λ_air / n = 632 nm / 1.50 = 421 nm .

💡 Key Knowledge

  • Refractive Index: Defined as n = c / v , where c is the speed in air/vacuum and v is the speed in the medium.
  • Wave Equation: v = f × λ .
  • Frequency Invariance: Frequency ( f ) is determined solely by the wave source and never changes upon refraction.
  • Wavelength Relationship: Because f is constant, v ∝ λ , which gives n = λ_air / λ_glass .

📐 Step-by-Step Calculation

  1. Identify known values:
    Wavelength in air, λ_air = 632 nm
    Refractive index of glass, n = 1.50
  2. State the relationship connecting wavelength and refractive index:
    Since n = c / v and c = f × λ_air while v = f × λ_glass :
    n = (f × λ_air) / (f × λ_glass) = λ_air / λ_glass
  3. Rearrange for wavelength in glass ( λ_glass ):
    λ_glass = λ_air / n
  4. Calculate the value:
    λ_glass = 632 nm / 1.50 = 421.33... nm ≈ 421 nm

🧠 Exam Technique & Quick Elimination

  • Sanity check the physics first: Glass is optically denser than air ( n > 1 ), meaning light slows down. Because speed decreases and frequency is unchanged, the wavelength must decrease ( < 632 nm ).
  • This instantly eliminates C (remains 632 nm) and D (948 nm, which is 632 × 1.50 ).
  • Between A (316 nm, which is 632 / 2 ) and B (421 nm), dividing 632 by 1.5 clearly yields approximately 420 nm, making B the obvious choice without tedious calculation.

❌ Common Distractor Traps

  • Option D (948 nm): The multiplication trap. Students multiply 632 × 1.50 instead of dividing, forgetting that light travels slower and compresses into shorter wavelengths in glass.
  • Option C (632 nm): The misconception that wavelength remains invariant across boundaries (confusing wavelength with frequency).
  • Option A (316 nm): Halving the wavelength ( 632 / 2 ) by mistakenly confusing a refractive index of 1.50 with a factor of 2.
Mark Scheme Breakdown: 1 mark awarded for selecting B (AO1 - recall and application of physical relationships).

Topics

Physics · 3.3 Waves

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.