In an experiment with a convex lens, The plot of the image distance $$\left(v^{\prime}\right)$$ against the object distance ($$\left.\mu^{\prime}\right)$$ measured from the focus gives a curve $v^{\prime} \mu^{\prime}=225$. If all the distances are measured in $\mathrm{cm}$. The magnitude of the focal length of the lens is $\underline{\hspace{2cm}}$ cm.