In the figure shown '$S$' is a monochromatic source of light emitting light of wavelength $\lambda$ (in air). Light falls on slit '${\mathrm{S}}_1$' from '$S$' and then reach the slits '${\mathrm{S}}_2$' and '${\mathrm{S}}_3$' 'through a medium of refractive index '${\mu }_1$'. Light from slits ${\mathrm{S}}_2$ and ${\mathrm{S}}_3$ reach the screen through medium of refractive index ${\mu }_3$. A thin transparent film of refractive index ${\mu }_2$ and thickness '$t$' is placed in front of '$S_2$'. Point '$P$' is symmetrical w.r.t. ${ \mathrm{S}}_2$ ' and '${\mathrm{S}}_3$' (Using the value $\mathrm{d}=1 \mathrm{mm},\mathrm{D}=1 \mathrm{m},{\mu }_1=4/3,{\mu }_2=3/2,{\mu }_3=9/5$ and $\mathrm{t}=\frac{4}{9}\times {10}^{-5} \mathrm{m}$ )