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In YDSE experiment, the $4^{\text {th }}$ dark band is formed opposite to one of the slits. The wavelength of light used is $(\mathrm{d}=$ distance between the slits, $\mathrm{D}=$ distance between source and the screen)
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The correct answer is:
$\frac{d^2}{7 D}$
Distance of $4^{\text {th }}$ dark band from the centre $=3.5 \frac{\lambda \mathrm{D}}{\mathrm{d}}$
$$
\begin{aligned}
& \therefore \frac{d}{2}=\frac{3.5 \lambda D}{d} \\
& \therefore \lambda=\frac{d^2}{7 D}
\end{aligned}
$$
$$
\begin{aligned}
& \therefore \frac{d}{2}=\frac{3.5 \lambda D}{d} \\
& \therefore \lambda=\frac{d^2}{7 D}
\end{aligned}
$$
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