In YDSE shown in figure a parallel beam of light is incident on the slits from a medium of refractive index `n_(1)`. The wavelength of light in this medium is `lambda_(1)`. A transparent slab of thickness t and refractive index `n_(3)` is put in front of one slit. The medium between the screen and the plane of the slits is `n_(2)`. The phase difference between the light waves reaching point O (symmetrical, relative to the slits) is

Explanation

`phi= (2pi)/(lambda_2)((n_3)/(n_2)-1)t= (2pi)/(n_(1)lambda_(1))n_(2)((n_(3)-n_(2))/(n_2))t`
`(therefore n_(1)lambda_(1)= n_(2)lambda_(2))" "phi= (2pi)/(n_(1)lambda_(1))(n_(3)-n_(2))t`.