When a light of wavelength 4000 A in vacuum travels through the same thickness in diamond and water separately, the difference in the number of waves is 200. Find the thickness, if refractive indices of diamond and water are $ \large\frac{5}{2}\: and \: \large\frac{4}{3}$ respectively :

Question

$\begin {array} {1 1} (1)\;0.685\: mm & \quad (2)\;0.0685\:mm \\ (3)\;68.5\: mm & \quad (4)\;6.85\: mm \end {array}$