Question

In the double-slit experiment, the distance of the first dark fringe from the central line is 1mm. The distance of the $2^{nd}$ bright fringe from the central line is

• A. 6mm
• B. 4mm
• C. 12mm
• D. 16mm

Answer 1

The correct option is B 4mm

$\begin{array}{cc}Brightband& Darkband\\ X_n=\frac{n\lambda D}{d}& X_n=(2n-1)\frac{\lambda D}{2d}\end{array}$
$n_B=2$
$n_D=1$
$\frac{X_{2B}}{X_{1D}}=\frac{n_B\times 2}{(2n_D-1)}=\frac{2\times 2}{2-1}$
$X_{2B}=4X_{1D}$
$=4\times 1mm$
$=4mm$