In Young's double slit experiment the slits, `S_(1) & S_(2)` are illuminated by a parallel beam of light of wavelength 4000 Å from the medium of refractive index `n_(1)=1.2` A thin film of thickness 1.2 `mum` and refractive index n=1.5 is placed infrom of `S_(1)` perpedicular to path of light. the refractive index of medium between plane of slits & screen is `n_(2)=1.4` if the light coming from the film and `S_(1)&S_(2)` have equal intensities I then intensity at geometrical centre of the screen O is

Options

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

Answer

2

Explanation

Phase difference at O: `(u_(rel)-1)t=(n/n_(2) -1)t`
`therefore` Phase difference at O: `(n/n_(2) -1)` where `n_(1)lambda_(1)=n_(2)lambda_(2) implies` Phase difference `=pi/2implies` Resultant intensity =21