A thin convex lens $L$ (refractive index $=1.5$ ) is placed on a plane mirror $M$. When a pin is placed at $A$, such that $\mathrm{OA}=18 \mathrm{~cm}$, its real inverted image is formed at A itself, as shown in figure. When a liquid of refractive index $\mu_1$ is put between the lens and the mirror, the pin has to be moved to $\mathrm{A}^{\prime}$, such that $\mathrm{OA}^{\prime}=27 \mathrm{~cm}$, to get its inverted real image at $\mathrm{A}^{\prime}$ itself. The value of $\mu$ । will be