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# Two monochromatic coherent sources of wavelength 5000 A are placed along line normal to screen as shown. Find the order of central bright fringe if $d = 0.5mm$ and $D = 1m$

(A) 500 $\qquad$ (B) 4000 $\qquad$ (C) 1000 $\qquad$ (D) 2000

At the central maxima, $\theta = 0$.
At the central maxima, $\theta = 0, \therefore p = d = n\;\lambda$
$\Rightarrow n = \large\frac{d}{\lambda}$
$\Rightarrow n = \large\frac{0.5}{5\times10^{-3}}$$= 1000$
edited Apr 19, 2014