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Question
Two slits spaced 0.25 mm apart are placed at 0.75 m from a screen and illuminated by coherent light with a wavelength of 650 nm.  The intensity at the centre of the central maximum (θ=0) is I0.  The distance on the screen from the centre of central maximum to the point where the intensity has fallen to I02 is nearly
Options:
A .  0.1 mm
B .  25 mm
C .  0.4 mm
D .  0.5 mm
Answer: Option D
:
D
I=I0cos2(πdsinθ)λ=I02
cos(πdsinθλ)=12
we get,
x=λD4d=650×109×0.754×0.25×103=487.5×106m
=0.4875mm=0.5mm

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