AS Daily A Level Physics question
In a lab double-slit experiment, a student projects fringes on a screen 2.0 m from slits separated by 0.50 mm using a 600 nm laser. They then change three things at once: they switch to a 500 nm laser, move the screen 20% further away, and swap in a double-slit slide with slit separation also 20% larger. Finally, they place a thin coverslip over just one slit that delays that path by an extra half-wavelength for the 500 nm light. The student had marked the position of the original central bright on the screen. Which statement must be true for the new pattern relative to that mark?
Answer
The correct answer is D.
Correct: D — Fringe spacing decreases by about 17%; the previously central bright point becomes dark. The fringe spacing scales with wavelength × screen distance ÷ slit separation, so increasing L and d by the same 20% cancels, and changing 600 nm to 500 nm multiplies spacing by 500/600 ≈ 0.83 (about 17% smaller); adding a half-wavelength delay to one slit makes the central point (equal geometric paths) have a net half-wavelength path difference, giving a dark. A is wrong because the wavelength change means spacing cannot stay the same, and the half-wave delay to one slit makes the old center dark, not bright. B is wrong because although the 17% decrease is correct, the half-wavelength delay makes the old central point a minimum, not a maximum. C is wrong because the spacing does not increase (500/600 < 1), even though the half-wave delay would indeed make the old center dark. D is correct for both the spacing change and the shift from bright to dark at the marked point.