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

$β=\frac{λd}{d}$

$β_1-β_2=\frac{λ(D_1-D_2)}{d}$ or $λ=\frac{d(β_1-β_2)}{D_1-D_2}$

or $λ=\frac{3×10^{-5}×10^{-3}}{5×10^{2}}=6000 Å$