(a) Light from a He-Ne laser at wavelength 633 nm illuminates two thin parallel slits with a separation of 2.50 mm and when young's interference fringe on screen is 1.3mm, distance of z=5.13m.
Now place a thin film of dielectric material of refractive index 1.46 and thickness 5.50 micrometer is now placed over one of the slits (say the right-hand one). By how much will this change the optical path between the slit and the centre of the screen?