AQA A-Level Physics Paper 3 (3BD), June 2025: Question 3

6 marks · Medium difficulty · Extended Answer

Discuss how the work of Fizeau, Foucault, Hertz and Maxwell contributed to our understanding of the nature of light, referencing pre-1800 models, Maxwell's theory, and the significance of their measurements.

Practise this question

Question

Question 03 is a 6-mark extended response question asking students to discuss how the work of Fizeau, Foucault, Hertz, and Maxwell contributed to the understanding of the nature of light. Four bullet points provide context: Fizeau measured the speed of light in air; Foucault confirmed light travels faster in air than water; Hertz measured the speed of radio waves in air; and Maxwell developed a theory used to obtain the speed of light. Students are instructed to refer to the two pre-1800 models for light, their predictions about the speed of light, Maxwell's theory, and the significance of the measurements, excluding experimental details.
Question text

03 The work of Fizeau, Foucault, Hertz and Maxwell contributed to our understanding of

the nature of light.

Fizeau measured the speed of light in air.

Foucault confirmed that light travels faster in air than in water.

Hertz measured the speed of radio waves in air.

Maxwell developed a theory that was used to obtain the speed of light.

Discuss how the work of these four scientists contributed to our understanding of the

nature of light.

In your answer you should refer to:

• the two pre-1800 models for the nature of light

• the predictions that these models made about the speed of light

• Maxwell’s theory

• the significance of the measurements made by Fizeau, Foucault and Hertz.

Do not include any experimental details in your answer.

[6 marks]

Mark scheme

Show the mark scheme Mark scheme for question 03 showing a level-of-response grid from 1 to 6 marks based on three main topic areas: Models before 1800, Maxwell's theory, and Significance. For Models before 1800, key points include Newton's corpuscular model versus Huygens' wave model, and their opposite predictions for the speed of light in a denser medium versus air. For Maxwell's theory, key points include predicting electromagnetic waves consisting of perpendicular oscillating electric and magnetic fields, and formula c = 1/sqrt(mu_0 epsilon_0). For Significance, points include Foucault showing the wave model was correct, Fizeau's measured speed matching Maxwell's prediction showing light is an EM wave, and Hertz confirming the speed of radio waves matched electromagnetic theory.

Question Answers Additional comments/Guidance Mark AO

Models before 1800

03 The mark scheme gives some guidance as to what 6 4 × AO1

1 bullet partially covers this area, all 3 are

statements are expected to be seen in a 1- or 2-mark (L1),

required to fully cover this area. 2 × AO2

3- or 4-mark (L2) and 5- or 6-mark (L3) answer. Guidance

• (Newton proposed) particle / corpuscular

provided in section 3.10 of the ‘Mark Scheme Instructions’

document should be used to assist in marking this model and (Huygens proposed) wave

question. model.

• Newton / particle / corpuscular model

Mark Criteria predicted that the speed of light in

All three areas covered in some detail. glass/water was faster than in air.

• Huygen / wave model predicted that the

6 marks can be awarded even if there is an

speed of light in glass/water was slower

error and/or parts of one aspect missing.

than in air.

5 All three areas covered, at least two in [Allow a comparison based on (optical)

detail. density.]

Whilst there will be gaps, there should only

be an occasional error. Maxwell’s theory

1 bullet partially covers this area, all 3 are

4 Two areas successfully discussed, or one

required to fully cover this area.

discussed and two others covered

• Idea that Maxwell predicted the existence of

partially. Whilst there will be several gaps,

/ the speed of electromagnetic waves.

there should only be an occasional error.

• Maxwell’s waves consisted of (oscillating)

3 One area discussed and one discussed electric fields and magnetic fields.

partially, or all three covered partially. There • At least one extra detail: e,g. Maxwell wrote

are likely to be several errors and omissions equations that related magnetic fields to

in the discussion. 1

electric fields / 𝑐 = where 𝜇0 is related

√𝜇0𝜀0

2 Only one area discussed, or makes a partial to magnetic field strength and 𝜀 is related

attempt at two areas. to electric field strength / electric field and

1 None of the three areas covered without magnetic field oscillates perpendicular to

significant error. each other and the direction of energy

transfer / electric and magnetic field

oscillate in phase …

– A-LEVEL PHYSICS – –

[Ignore the idea that Maxwell predicted the

0 No relevant comments speed of light.]

Significance

1 bullet partially covers this area, 2 is enough

to fully cover this area.

• Foucault’s experiment suggested

wave/Huygen model was correct /

particle/Newton/corpuscular model was

incorrect.

• Fizeau’s measurement matched the

prediction of the speed of electromagnetic

waves suggesting that light was an

electromagnetic wave.

• Hertz's measurement equalling predicted

speed of electromagnetic waves confirmed

Maxwell’s theory.

[Ignore idea that Hertz’s measurement

matching Maxwell’s prediction showed radio

waves were electromagnetic.]

[Experiments given in question, so answer

must link these experiments to their

significance not just speeds matching.]

Total 6

How to answer it

Historical Evolution of the Nature and Speed of Light

AQA Turning Points in Physics • Extended 6-Mark Question

What this question tests

This question evaluates your understanding of how experimental evidence shaped the theory of light from pre-1800 models through to electromagnetic theory. Key aspects assessed include:

  • Pre-1800 models: Newton’s corpuscular model vs. Huygens’ wave model, and their contrasting predictions for light speed in optically denser media (water/glass vs. air).
  • Maxwell's theoretical prediction: The nature of electromagnetic waves, mutual perpendicularity of self-propagating fields, and the equation relating speed to fundamental constants c = 1 / √(μ₀ε₀) .
  • Significance of experiments: Fizeau’s measurement validating Maxwell’s calculated speed, Foucault’s proof discrediting Newton, and Hertz’s discovery confirming the EM spectrum.
  • Synoptic structure: Following the prompt’s explicit restriction: Do not include experimental details.

Question 03: 6-Mark Level Descriptor Breakdown

Level Marks Requirement Across the Three Core Areas
Level 3 5 – 6 All three areas covered in detail (at least two fully in-depth for 5 marks; comprehensive coverage with minimal omissions for 6 marks).
Level 2 3 – 4 Two areas successfully covered, or one covered well and two partially covered. Minor omissions allowed.
Level 1 1 – 2 Only one area discussed, or partial attempts across areas with significant errors/gaps.

Area 1: Pre-1800 Models and Their Predictions

Corpuscular Model vs. Wave Theory

✅ Key Points to State

  • Newton proposed the particle (corpuscular) model of light.
  • Huygens proposed the wave model of light.
  • Newton's prediction: Light travels faster in optically denser media (water/glass) than in air, because an attractive force pulls corpuscles into the medium, increasing their normal velocity component.
  • Huygens' prediction: Light travels slower in optically denser media (water/glass) than in air to produce refraction towards the normal.

🧠 Exam Technique

Pair each scientist directly with their model and explicit speed prediction in denser media. A comparative structure guarantees you satisfy all requirements for this area.

Newton → Corpuscular → Faster in water
Huygens → Wave → Slower in water

Area 2: Maxwell's Theoretical Breakthrough

Prediction and Nature of Electromagnetic Waves

💡 The Core Physics

  • Maxwell unified electricity and magnetism, predicting the existence of electromagnetic waves.
  • These waves consist of oscillating electric fields (E) and magnetic fields (B) perpendicular to each other and perpendicular to the direction of propagation (transverse wave).
  • The oscillations are in phase with one another.
  • He derived the theoretical speed using fundamental constants of free space:
    c = 1 / √(μ₀ε₀)
  • Where μ₀ is the permeability of free space and ε₀ is the permittivity of free space.

❌ Common Examiner Traps

  • Vague descriptions: Saying Maxwell "measured" or "discovered" the speed. He did not measure it; he calculated it theoretically from electrical and magnetic constants.
  • Missing field orientation: Forgetting to mention that electric and magnetic fields oscillate mutually at 90° (perpendicular) to each other and to the wave velocity.

Area 3: Significance of Experimental Measurements

Fizeau, Foucault, and Hertz

✅ What Each Experiment Proved

  • Foucault: Measured the speed of light in water and confirmed it was slower than in air. This definitively disproved Newton’s corpuscular model and strongly supported Huygens' wave theory.
  • Fizeau: Made the first terrestrial measurement of the speed of light in air (~3.1 × 10⁸ m s⁻¹). This matched Maxwell's calculated value of c = 1 / √(μ₀ε₀) , suggesting light is an electromagnetic wave.
  • Hertz: Produced and detected radio waves, showing they had the exact same speed as light and underwent reflection and refraction, confirming Maxwell's electromagnetic theory across the spectrum.

❌ Lost Marks to Avoid

  • Describing apparatus: The prompt explicitly states: "Do not include any experimental details in your answer." Describing toothed wheels, rotating mirrors, or spark gap circuits wastes time and earns zero marks.
  • Stopping at speeds match: Simply repeating that Hertz found the speed was 3 × 10⁸ m s⁻¹ gets no credit. You must state the significance: it confirmed Maxwell's prediction that EM waves existed beyond visible light.

Top-Level Model Answer Structure (6/6 Marks)

📝 Full Marks Response Plan

  1. Pre-1800 Models: Newton proposed the corpuscular (particle) theory, predicting light travels faster in dense media like water. In contrast, Huygens proposed the wave theory, predicting light travels slower in dense media.
  2. Significance of Foucault: Foucault experimentally showed light travels slower in water than in air. This disproved Newton’s corpuscular model and confirmed Huygens’ wave model.
  3. Maxwell’s Theory: Maxwell showed that oscillating electric and magnetic fields propagate together at right angles to each other as an electromagnetic wave. He derived the wave speed from electrical and magnetic constants as c = 1 / √(μ₀ε₀) .
  4. Significance of Fizeau: Fizeau measured the speed of light in air, which matched Maxwell’s theoretical calculation. This provided strong evidence that light itself is an electromagnetic wave.
  5. Significance of Hertz: Hertz generated and detected radio waves, measuring their speed to be identical to light. This experimental evidence verified Maxwell’s electromagnetic theory.

Topics

Optional topics · 3.12 Turning points in physics (A-level only)

Question and mark scheme from the AQA A-Level Physics examination, Paper 3 (3BD), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.