Q30. 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 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 $4 ^ { \text {th } }$ bright fringe. The thickness of the glass-plate is $\_\_\_\_$ $\mu \mathrm { m }$.