Q.What must be the thickness of a thin film which, when kept near one of the slits shifts the central fringe by 5 mm for incident light of wavelength 5400 Å in Young's double slit interference experiment? The refractive index of the material of the film is 1.1 and the fringe width is 0.5 mm.

The film adds an optical path (n − 1)t; equate it to the extra geometric path yd/D = yλ/W that the shift correspon …

The film introduces an extra optical path (n − 1)t = (1.1 − 1)t = 0.1t. The central fringe shifts to the point where this equals the extra geometric path difference, which for a shift y is yd/D = yλ/W: …

Set the film's extra optical path (n−1)t equal to yλ/W (the extra path f …

Using nt instead of (n−1)t for the extra optical path — the film REPLACES a thickness …