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White light is used in a YDSE with $D= 1m$ and $d= 0.9 mm$ .Light reaching the screen at position $y= 1 mm$ is passed through a prism and its spectrum is obtained. The missing lines in the visible region of this spectrum is :-

$(a)\;300\;nm \\ (b)\;600\;nm \\ (c)\;both \;a\; and\; b \\ (d)\;None $

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1 Answer

$\Delta P= \large\frac{yd}{D}$
$\qquad= 900 nm$
for minima $\Delta P = \large\frac{(2n-1) \lambda}{2}$
=> $\lambda =\large\frac{2 \Delta P}{(2n-1)}=\frac{1800}{2n-1}$
$\qquad=\large\frac{1800}{1} , \frac{1800}{3}, \frac{1800}{5}, \frac{1800}{7}$
of these $600,360\;nm$ lie in visible region.
Hence these will be missing lines in the visible spectrum.
Hence c is the correct answer.
answered Feb 13, 2014 by meena.p

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