Wave Optics Part 2

Monochromatic light of wavelength 500‌nm is used in Young's double slit experiment. An interference pattern is obtained on a screen When one of the slits is covered with a very thin glass plate (refractive index =1.5 ), the central maximum is shifted to a position previously occupied by the 4‌th ‌ bright fringe. The thickness of the glass-plate is .............µm.