Physics - Wave Optics Question with Solution | TestHub

In Young’s double slit experiment, a fringe width$=\frac{\lambda D}{d}$

Given, that$x_2-x_1=12\frac{{\lambda }_{1}D}{d}\mathrm{...}(i)(where{\lambda }_1=600nm)$

Also,$x_2-x_1=k\frac{{\lambda }_{2}D}{d}$(Where k is the number of fringes)

Dividing (i) and (ii)



$1=\frac{12{\lambda }_1}{k{\lambda }_2}\therefore k=\frac{12\times 600}{400}=18$

$\therefore$  The number of fringes observed now = 18.