AQA A-Level Physics Paper 1, November 2020: Question 17

1 mark · Medium difficulty · Multiple Choice

Identify the next three values of angle theta in radians for which unpolarised light passing through two polarising filters hits the screen with maximum intensity when one filter is rotated.

Practise this question

Question

A multiple-choice question showing unpolarised light passing through polarising filters X and Y to a screen. A diagram illustrates filter X held stationary and filter Y rotated by an angle theta. Four options A, B, C, and D give different sequences of angle values in radians for maximum intensity.
Question text

17 Unpolarised light travels through two polarising filters X and Y and is then incident on a

screen. When X and Y are arranged as shown, there is a maximum intensity on the

screen.

X is held stationary but Y is rotated in a plane at right angles to the beam so that

θ increases.

What are the next three values of θ, in rad, for which the beam hits the screen with

maximum intensity?

[1 mark]

π 2π 3π

A , ,

22 2

π 3π 5π

B , ,

22 2

C π, 2π, 3π

D 2π, 4π, 6π

Mark scheme

Show the mark scheme The mark scheme table shows question 17 has the correct answer option C.

17 C

How to answer it

Question 17: Rotation of Polarising Filters

What this question tests

This question assesses your understanding of wave optics, specifically the transmission of unpolarised light through consecutive polarising filters, Malus's law, and the angular periodicity (symmetry) of transmitted light intensity as a polarising filter is rotated through 360 degrees (2π radians).

Question 1.7 • Multiple Choice

Part (1.7) • Identifying Subsequent Maximum Intensity Angles

✅ Correct Answer

C (π, 2π, 3π)

1 mark

💡 Key Knowledge

  • Unpolarised light passing through filter X becomes plane-polarised.
  • The intensity transmitted through a second filter Y is governed by Malus's law: I = I₀ cos²(θ) .
  • Maximum intensity occurs when cos(θ) = ±1, which happens every π radians (180°), not every 2π radians.

🧠 Exam Technique

Look carefully at how angle θ is defined in the diagram. Notice the dashed reference line showing the initial alignment. As filter Y rotates, the transmission axis aligns twice per complete revolution (at 0, π, 2π, 3π, etc.) because polarising filters possess 180° rotational symmetry regarding light transmission.

❌ Common Errors

Students frequently select option D (2π, 4π, 6π) or option B (π/2, 3π/2, 5π/2) by confusing complete cycles of filter rotation with the periodicity of light intensity. Intensity peaks twice per full rotation of 2π.

Topics

Physics · 3.3 Waves

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.