A transparent paper (refractive index = 1.45) of thickness 0.02 mm is pasted on one of the slits of a Young’s double slit experiment which uses monochromatic light of wavelength 620 nm. How many fringes will cross through the center if the paper is removed?
Given, the thickness of the sheet, 
.
Wavelength of light, 
refractive index, 
Therefore the optical path difference, 
If there was no film present, there is no path difference at the center. But due to the presence of the film, we have a path difference of 
.
Now for path difference of 
we have 1 fringe shift. Thus for a path difference of 
, we have
fringe shifts.
or, 
2