In the figure shown in a YDSE, a parallel beam of light is incident on the slits from a medium of refractive index n1 . The wavelength of light in this medium is λ1 . A transparent slab of thickness ' t ' and refractive index n3 is put in front of one slit. The medium between the screen and the plane of the slits is n2 . The phase difference between the light waves reaching point O (Symmetrical, relative to the slits) is:
