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In Young's double slit, $\lambda = 600$ nm, slit separation $d = 0.5$ mm, screen distance $D = 1$ m. Fringe width is:

A$0.3$ mm, ignoring the slit separation in the formula here
B$1.2$ mm, by $\beta = \lambda D/d$ with the given values
C$3$ mm, since fringe width scales with $\lambda D \cdot d$ here
DZero, since coherent slits give a single bright point only
Answer & Solution
Correct answer: B. $1.2$ mm, by $\beta = \lambda D/d$ with the given values
$\beta = \lambda D/d = 600\times 10^{-9}\cdot 1/0.5\times 10^{-3} = 1.2\times 10^{-3}$ m = 1.2 mm.
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