In a Young's double slit experiment the slits `S_(1)`& `S_(2)` are illuminated by a parallel beam of light of wavelength `4000 overset(o)(A)`, from the medium of refractive index `n_(1) = 1.2`. A thin film of thickness `1.2mum` and refractive index `mu` = 1.5 is placed infront of perpendicular to path of light. The refractive index of medium between plane of slits & screen:

Options

A. 0
B. 2I
C. 4I
D. None of these

Answer

B

Explanation

Path difference at O: `(mu_(rel)-1)t=((n)/(n_(2))-1)t`
`therefore `phase difference at O: `t((n)/(n_(2))-1)(pi)/(lamda_(2))` where `n_(1)lamda_(1)-n_(2)lamda_(2)`
`implies` phase difference `=(pi)/(2)implies"Resultant intensity "=2I`