Back to Daily Question archive

A2 Daily A Level Physics question

2026-02-27 OCR A Exam practice set A: misconceptions clinic — path difference Module 4.4 Waves — Interference: Young’s double-slit; path difference and fringe spacing (y ∝ λL/d) Module 4.4 Waves — Wave properties: phase difference; relationship between f and λ for constant wave speed

In a lab double-slit setup, the slit separation is d = 0.50 mm. With a red laser of wavelength 640 nm and a screen at L = 2.0 m, a pencil mark is made at the centre of the m = +3 bright fringe, 7.7 mm above the central maximum. The laser is then replaced by one of double frequency, and the screen is moved to L = 1.0 m; alignment is unchanged and the angles are small. Which statement must be true about what is now at the pencil mark?

  1. A It is on the m = +6 bright fringe; halving the screen distance halves the spacing while doubling frequency doubles the number of fringes across the screen.
  2. B It is on a dark fringe at that position, because halving the wavelength turns a previous bright point into a destructive condition.
  3. C It is again the m = +3 bright fringe, because halving the wavelength and halving the screen distance cancel to leave the spacing unchanged.
  4. D It is on the m = +12 bright fringe, because the bright-fringe spacing is reduced by a factor of four. (correct)

Answer

The correct answer is D.

Correct: D — It is on the m = +12 bright fringe, because the bright-fringe spacing is reduced by a factor of four. Doubling frequency halves the wavelength, and halving L also halves the spacing; together the spacing scales as λL, so it becomes one quarter, making the mark that was at 3 spacings now at 12 spacings (still bright on the same side; small-angle is valid since y/L ≪ 1). A … Underestimates the change: treating the two effects as giving only a factor of two, but λ and L each halved make the spacing one quarter, not one half. B … Confuses wavelength change with a bright–dark flip; at the mark the new order is an integer (m = 12), so it remains a bright fringe. C … Incorrectly claims the effects cancel; halving λ and halving L do not cancel but multiply to reduce spacing by a factor of four. D … Correct as shown by proportional reasoning: m′ = y/Δy′ = y/(Δy/4) = 4(y/Δy) = 12.

Try today's guest Daily MCQ A Level Physics MCQ practice Problem-solving previews