In a two slit experiment with monochromatic light, fringes are obtained on a screen placed at some distance from the slits. If screen is moved by 5 × 10^–2 m towards the slits, then change in fringe width is 3 × 10^–5 m. If the distance between slits is 10^–3 m then wavelength of the light used will be

6000 Å 

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

$\beta_1 - \beta_2 = \frac{\lambda(D_1 - D_2)}{d}$

$ \lambda = \frac{d(\beta_1 -\beta_2)}{\lambda(D_1 - D_2)} = \frac{3\times 10^{-5}\times 10^{-3}}{5\times 10^2} = 6000A^o$