A contact lens is in the shape of a convex meniscus (see Fig. 31.25). The inner surface is curved to fit the eye, with curvature radius $7.80 \mathrm{~mm}$. The lens is made from plastic with refractive index $n=1.56$. If it has a $44.4-\mathrm{cm}$ focal length, what's the curvature radius of its outer surface?