A) 0.45 mm
B) 0.40 mm
C) 0.30 mm
D) 0.20mm
Correct Answer: D
Solution :
The intensity at distance x-from central order bright on screen is \[I={{I}_{0}}{{\cos }^{2}}\frac{\pi x}{\beta }\]where \[{{I}_{0}}=\]maximum intensity \[\beta =\]fringe width If \[I=\frac{3}{4}{{I}_{0}}\]\[\Rightarrow \]\[\cos \frac{\pi x}{\beta }=\frac{\sqrt{3}}{2}=\cos \frac{\pi }{6}\]\[\therefore \]\[x=\frac{\beta }{6}\]Hence the required distance \[=2\times \frac{\beta }{6}=\frac{\beta }{3}=0.20\,\,mm\]
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