In a standard YDSE setup, the fringe width on the screen is 1.5mm. When a thin glass film is pasted in front of the upper slit, the fringe pattern shifts up. But it is seen that a point P above central maxima where intensity was one fourth the intensity at central maxima, intensity remains the same. There were no maxima between central maxima and point P before film was introduced. What can be the thickness of the film? Take $\mu =15,\lambda =450nm$ :-