Monochromatic light of wavelength 500 nm is used in Young's double slit experiment. An interference pattern is obtained on a screen. When one of the…

Monochromatic light of wavelength $\displaystyle 500$ nm 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 $\displaystyle =1.5$ ), the central maximum is shifted to a position previously occupied by the $\displaystyle 4{ }^{\text {th }}$ bright fringe. The thickness of the glass-plate is $\displaystyle \_\_\_\_$ $\displaystyle \mu \mathrm{m}$.