In two separate set ups of Young's double slit experiment, fringes of equal width are observed when lights of wavelengths in the ratio 1:2 are used. If the ratio of slit separation in two cases is 2:1, then ratio of distances between the plane of slits and the screen in the two set ups is

4 : 1

$β=\frac{λ_1D_1}{d_1}=\frac{λ_2D_2}{d_2}$

$∴\frac{D_1}{D_2}=\frac{d_1}{d_2}×\frac{λ_2}{λ_1}=\frac{2}{1}×\frac{2}{1}=4:1$