A2 Daily A Level Physics question
In a Young’s double-slit lab setup using monochromatic light, a thin, uniform transparent film is placed over just one slit without disturbing the geometry. Which statement must be true about the interference pattern observed on a distant screen?
Answer
The correct answer is C.
Correct: C — The entire fringe system shifts towards the slit covered by the film, with fringe spacing essentially unchanged. The film increases the optical path for that slit, introducing a constant phase delay; maxima then occur where the geometrical path from the covered slit is slightly shorter, i.e. closer to it, so the whole pattern shifts without altering the spacing set by wavelength in air, slit separation and screen distance. A is wrong because the direction is reversed and the spacing would not increase; the film does not change slit separation or the wavelength in air. B is wrong because the central maximum does not stay fixed when a constant phase is added to one path, and the spacing does not decrease. C is correct as explained. D is wrong because a uniform, thin, transparent film primarily adds phase; with negligible loss the contrast need not change, and in any case a positional shift does occur.