If I do the double-slit experiment underwater, do the fringes change?
Yes.
Yes. Water shrinks the wavelength to λ' = λ/n (n ≈ 1.33), and fringe width β = λ'D/d shrinks with it. So the whole pattern contracts — fringes move closer together by a factor of n. The colours and slit geometry are the same; only the wavelength (and hence fringe spacing) is reduced by the medium.
Key point
In a medium, λ → λ/n, so fringe width β shrinks by the refractive index.
Common mistake
assuming the medium doesn't affect the fringe pattern.
Memory tip
denser medium → smaller λ → tighter fringes.
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