The physics
Light from two slits reaches each spot on the screen by two paths. Where the paths differ by a whole number of wavelengths, the spot is bright.
path difference = y d / D
So the bright fringes sit at equal gaps. That gap is the fringe width β. Halfway between two bright fringes sits a dark one.
β = λD / d bright: y = mβ dark: y = (m − ½)β
In a liquid of index μ, the wavelength drops to λ/μ. The fringes shrink by the same factor.
Two colours are bright together where m₁λ₁ = m₂λ₂. A biprism acts like two slits 2a(μ − 1)α apart.