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# A Young's double slit experiment is done in air and the same set up is immersed in a liquid of refractive index $\mu$. It is found that the 10th bright fringe in liquid was in the same position as 6th dark fringe in air . The value of $\mu$ is

$(a)\;1.52 \\ (b)\;1.8 \\ (c)\;1.65 \\ (d)\;1.5$

Let $\beta$ be fringe width in air and $\beta'$ be fringe width in liquid.
Since $10th$ bright fringes in liquid coincide with 6 dark fringe in air.
$10\;\beta'=5.5 \beta$
Also, $\large\frac{\beta}{\beta'}$$=\mu$
$\therefore \mu =\large\frac{10}{5.5}$
$\qquad= 1.8$