In a Young’s double slit experiment, slits are separated by $0.5\; mm$, and the screen is placed $150\; cm$ away. A beam of light consisting of two wavelengths, $650 \;nm$ and $520\; nm$, is used to obtain interference fringes on the screen. The least distance from the common central maximum to the point where the bright fringes due to both the wavelengths coincide is :

Question

$\begin{array}{1 1} (1) 1.56 mm \\ (2) 7.8 mm \\ (3) 9.75 mm \\ (4) 15.6 mm \end{array}$