A two slit Young's experiment is done with monochromatic light of wavelength 6000 Å. Slits are 2 mm apart and fringes are observed on a screen placed 10 cm away from the slits. If a transparent plate of thickness 0.5 mm is placed in front of one of the slit, interference pattern shifts by 5 mm. Then refractive index of transparent plate should be

1.2

$(μ-1)t=\frac{xd}{D}$ or $μ=1+\frac{xd}{Dt}$

or $μ=1+\frac{0.5×0.2}{10×0.05}=1+0.2=1.2$