A2 Daily A Level Physics question
In a lab double-slit experiment using a laser, the 5th bright fringe is 12 mm from the central maximum on a screen 2.0 m away. The slit separation is then reduced by 20%, with wavelength and screen distance unchanged. What is the new distance of the 5th bright fringe from the central maximum? Explain using how fringe position scales with slit separation.
Answer
The correct answer is B.
Correct: B — 15 mm. Fringe position for a given order scales inversely with slit separation, so reducing d by 20% (to 0.8d) makes the fringe distance larger by a factor 1/0.8 = 1.25; 12 mm × 1.25 = 15 mm. A assumes the distance scales directly with d (12 × 0.8 = 9.6 mm), which reverses the true dependence. B is correct as it applies the inverse proportionality, giving a 25% increase. C assumes only a 20% increase (12 × 1.2 = 14.4 mm), ignoring that the change is reciprocal, not additive. D suggests no change, overlooking that fringe spacing does depend on slit separation when wavelength and screen distance are fixed.