AS Daily A Level Physics question
A red laser (fixed frequency) illuminates a double-slit, forming clear fringes on a screen a fixed distance away. The entire apparatus (laser, slits and screen) is then submerged in water without moving any parts. Which statement about the fringe spacing on the screen must be true, and why?
Answer
The correct answer is A.
Correct: A — The fringes get closer together because the wavelength in water is smaller while the laser frequency stays the same. B is wrong because a lower wave speed in water leads to a smaller wavelength (with the same source frequency), which reduces fringe spacing rather than increasing it. C is wrong because it ignores that fringe spacing depends on the wavelength in the medium as well as geometry; the wavelength decreases in water. D is wrong because the source frequency does not change when the medium changes; it is the speed and hence the wavelength that change.