A2 Daily A Level Physics question
In a darkened lab, a student observes a double-slit interference pattern on a screen 2.0 m away. With slit separation 0.50 mm and a 633 nm red laser, the distance from the central bright fringe to the first-order bright fringe is 2.5 mm. The student then uses slits of separation 0.40 mm and a 532 nm green laser, with everything else unchanged. What happens to the first-order fringe separation from the centre?
Answer
The correct answer is B.
Correct: B — It increases by about 5% to around 2.6 mm. Fringe spacing is proportional to wavelength and inversely proportional to slit separation at fixed screen distance, so the ratio new/old ≈ (532/633) × (0.50/0.40) ≈ 0.84 × 1.25 ≈ 1.05, giving about a 5% increase (2.5 mm → ≈2.6 mm). A Decreasing is the wrong sign: reducing slit separation tends to increase spacing more than the shorter wavelength reduces it, so the net change is a slight increase, not decrease. C Doubling is far too large: the combined factor is about 1.05, not 2; this confuses proportional changes with a full doubling. D Unchanged ignores the competing effects of changing wavelength and slit separation; they do not cancel exactly (net ≈ +5%).