In a young's double slit experiment, slits are illuminated by a monochromatic source of wavelength 600nm and fringes are obtained. If screen is moved by a distance of 5 cm towards slits, change in fringe width is 3 x 10^–5 m. Then separation between the slits will be 

1 mm                

$ \beta = \frac{\lambda D}{d} , \beta'= \frac{\lambda D'}{d}$

$ \beta - \beta'= \frac{\lambda}{d} (D - D') $

$ d = \lambda \frac{D - D'}{\beta - \beta'} = 6\times 10^{-7}\frac{5\times10^{-2}}{3\times 10^{-5}} = 1mm$