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

$ \beta = \frac{\lambda_1D_1}{d_1} = \frac{\lambda_2D_2}{d_2} $

$ \frac{D_1}{D_2} = \frac{d_1}{d_2}\frac{\lambda_2}{\lambda_1} = \frac{2}{1} \frac{2}{1} = 4:1$