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# In a YDSE , 12 fringes are observed to be formed in a certain segment of screen when light of wavelength $600\; nm$ is used. If the wavelength of length is changed to $400\; nm$ , number of fringes observed in same segment of screen in given by

$(a)\;12 \\ (b)\;18 \\ (c)\;24 \\ (d)\;30$

No of fringes $= \large\frac{ length \;of\; region}{fringe\; width}$
We know that
Fringe width $\alpha$ wavelength
$12 \times 600=x \times 400$
$x=18$
Hence b is the correct answer.