AQA AS Level Physics Paper 2, November 2021: Question 19

1 mark · Medium difficulty · Multiple Choice

Identify which change decreases the fringe separation in a Young's double-slit experiment.

Practise this question

Question

Multiple choice question 19 asking which change decreases fringe separation in a double-slit experiment, with four options A to D involving slit separation, distance to screen, frequency, and wavelength.
Question text

19 In a Young’s double-slit experiment, monochromatic light is incident on two narrow slits

and the resulting interference pattern is observed on a screen.

Which change decreases the fringe separation?

[1 mark]

A decreasing the separation between the two slits

B increasing the distance between the slits and the screen

C using monochromatic light of higher frequency

D using monochromatic light of longer wavelength

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C: using monochromatic light of higher frequency.

19 C using monochromatic light of higher frequency

How to answer it

Young's Double-Slit Experiment Fringe Separation

AQA AS Level Physics • Waves • Multiple Choice

What this question tests

This question assesses your understanding of wave interference, specifically how changes to experimental parameters affect the fringe separation in a Young's double-slit setup. It tests your ability to recall and manipulate the fringe spacing equation, link wave frequency to wavelength, and deduce proportional relationships under exam conditions.

Question 19 Analysis

Identifying the correct proportional relationships

✅ Correct Answer: C

Using monochromatic light of higher frequency decreases the wavelength (since c = fλ), which in turn decreases the fringe separation w because w is directly proportional to λ.

💡 Key Knowledge

The fringe separation formula is given by: w = λs / D (often expressed as w = λD / s , where s or d is slit separation and D is distance to the screen). Remember that frequency and wavelength are inversely proportional ( c = fλ ).

🧠 Exam Technique

Multi-step proportional questions like this require breaking the change down into two logical jumps: 1. How does the altered variable affect wavelength? 2. How does that new wavelength affect the fringe spacing?

❌ Common Errors

A frequent trap is confusing inverse and direct proportions—for example, thinking higher frequency means longer wavelength, or assuming decreasing slit separation ( s ) decreases fringe spacing when it actually increases it.

Mark Scheme Note: 1 mark awarded for selecting option C. No working out required for multiple-choice questions, but quick mental substitution prevents careless errors.

Topics

Physics · 3.3 Waves

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.