In the Young’s double slit experiment, separation between the two slits is 0.9 mm, and the fringes are observed one metre away, if it produces the second dark fringe at a distance of 1 mm from the centreal frings, then wavelength of the monochromatic source of light used is

Given d = 0.9 mm =$0.9\times {10}^{-3}m$  ; D = 1 m;
Distance between the second dark fringe and central fringe $\left(x_2\right)=1mm={10}^{-3}$m and number of dark fringe $\left(n\right)=2$. We know from the Young’s double slit experiment that distance between second dark fringe and central fringe $x_2=\frac{\left(2n-1\right)D\lambda }{2d}$ or
 $\lambda =\frac{2d{x}_2}{\left(2n-1\right)D}$ = $\frac{2\times \left(0.9\times {10}^{-3}\right)\times {10}^{-3}}{\left[\left(2\times 2\right)-1\right]\times 1}$
= $6\times {10}^{-7}m=60mm$
Hence, the correct answer is option (d).