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