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# In a young's double slit experiment 'd' distance between slit is $0.5\; mm$ and illuminated by light of $\lambda =6000 A^{\circ}$ If screen distance is 2.5m ,thedistance of third bright fringe from centre.

$(a)\;1.5 \;mm \\ (b)\;3\;mm \\ (c)\;2.5\;mm \\ (d)\;9\;mm$

Distance of 3rd maxima from centre.
$y_n= \large\frac{n \lambda D}{d}$
$y_3 =\large \frac{3 \times 6000 \times10^{-10} \times 2.5 }{0.5 \times 10^{-3}}$
$\qquad= 9 \times 10^{-3}\;m$
$\qquad= 9 mm$
Hence c is the correct answer.