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IB · C · multiple choice

Wave behaviour · Question 5

Wave behaviour · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
demanding
Marks
1
Topics
1
Answer
Complete
Monochromatic light passing through a single slit onto a distant screenmonochromatic lightsingle slit of width bscreen
Figure 3Plan view of the arrangement. Three parallel rays of monochromatic light travel from the left towards an opaque barrier drawn as two vertical bars; only the middle ray meets the narrow gap between them, which is labelled 'single slit of width b'. Beyond the slit, two faint rays spread out above and below a dashed straight line that continues from the slit to a screen at the far right. On the screen a row of bright bands is drawn, centred on the dashed line, with the band on the line noticeably taller than the pairs of bands above and below it.
multiple choice1 mark

Monochromatic light passes through a single slit and forms a diffraction pattern on a distant screen. The slit is then made narrower. What happens to the width of the central maximum and to its intensity?

  1. A

    width increases, intensity decreases

  2. B

    width increases, intensity increases

  3. C

    width decreases, intensity decreases

  4. D

    width decreases, intensity increases

Ready to self-mark?Reveal the detailed answer guide
Answer overviewCorrect answer: A — width increases, intensity decreases
01

Answer and reasoning

1 mark
How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

  1. 1

    The correct choice is A: width increases, intensity decreases

  2. 2

    C is the intuitive answer — a narrower slit ought to give a narrower patch — and it is exactly backwards. The angular width of the central maximum is 2λ/b, so narrowing b spreads the pattern out, while less light passes through and the pattern is dimmer.

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