A screen is at a distance D = 80 cm from a diaphragm having two narrow slits $S_1$ and $S_2$ which are d = 2 mm apart. Slit $S_1$ is covered by a transparent sheet of thickness $t_1=2.5\mu m$ and $S_2$ by another sheet of thickness $t_2=1.25\mu m$ as shown in figure. Both sheets are made of same material having refractive index $\mu =\mathrm{1.40.}$ Water is filled in space between diaphragm and screen. A monochromatic light beam of wavelength $\lambda =5000\stackrel{\text{0}}{\text{A}}$ is incident normally on the diaphragm. Assuming intensity of beam to be uniform and slits of equal width, calculate ratio of intensity at C to maximum intensity of interference pattern obtained on the screen, where C is foot of perpendicular bisector of $S_1{S}_2.$ (Refractive index of water, ${\mu }_w=4/3$ ). If the ratio is 3/k, find ‘k’.