A liquid of refractive index ${\mu }_0$ is filled between slits and screen. Initially the central maxima is at point O on screen, with no slabs in front of slits. Now two slabs of refractive index ${\mu }_1$  and ${\mu }_2$ , with thickness $t_1$ and $t_2$ respectively are placed in front of slits as shown, So that the position of  central maxima does not change. Find the ratio of $\frac{t_1}{t_2}$  , if  ${\mu }_0=1.5,{\mu }_1=1.8and{\mu }_2=2.1$ .