In a Youngs double-slit experiment, the slits are separated by 0.28 mm and the screen is placed 1.4 m away. The distance between the central bright fringe and the fourth bright fringe is measured to be 1.2 cm. Determine the wavelength of light used in the experiment.

Distance between the slits, $d=0.28\times 10^{-3}m$

Distance between the slits and the screen, $D=1.4m$

Distance between the central fringe and the fourth $(n=4)$ fringe, 

$u=1.2\times 10^{-2}m$

In case of a constructive interference, we have the relation for the distance between the two fringes as:

$u=n\lambda D/d$

$\Rightarrow \lambda =ud/nD=6\times 10^{-7}m=600nm$