AS Daily A Level Physics question
A double-slit interference pattern is produced on a screen using monochromatic light in a vacuum. The entire experiment is then repeated with the apparatus submerged in a transparent liquid with a refractive index of n = 1.5. The geometry of the apparatus (slit separation and distance to the screen) is unchanged. How does the new pattern formed in the liquid compare to the original pattern in the vacuum?
Answer
The correct answer is A.
Correct: A — The fringe spacing decreases by a factor of 1.5, and the central maximum remains bright. The wavelength of light in a medium with refractive index n is reduced by that factor (λ_liquid = λ_vacuum / n). Since the fringe spacing (w) is directly proportional to the wavelength (λ) in the double-slit formula, the spacing also decreases by the same factor of 1.5. The central maximum corresponds to zero path difference, which always results in constructive interference, so it remains bright. B is incorrect because it reverses the effect; the wavelength and fringe spacing decrease, not increase, in a denser medium, and the central maximum remains bright. C is incorrect because the wavelength of light changes when it enters a different medium, which directly affects the fringe spacing. D is incorrect as it misapplies the proportional reasoning; the new first-order fringe is at a position x = (λ/1.5)D/s, whereas the original second-order fringe was at x = 2λD/s, which are not the same positions.