For a light of wavelength λ, the fringe width in double slit experiment is given by
β=dλD...(1)
For a light of wavelength λ′, the fringe width is given by
β′=dλ′D...(2)
Divide equation (2) by equation (1) and simplify to obtain the fringe width for the later case.
ββ′==⇒dλDdλ′Dλλ′β′=λλ′β...(3)
Substitute the values of the known parameters into equation (3) to calculate the required fringe width.
β′==400nm600nm×2mm3mm