Monochromatic light of wavelength 500nm is used in a Young’s double-slit experiment with slit separation 0.25mm and screen distance 1.0m. A glass plate of refractive index 1.40 and unknown thickness t is introduced in front of one slit. The screen is then moved parallel to itself by 2.0mm towards that slit so that the central bright fringe again coincides with its original mark. Derive an expression relating t to the given quantities and calculate its value.