SPM is a spherical refracting surface separating two media of refractive indices `n_1` and `n_2`. Consider an object point O forming a virtual image at the point I. We assume that all rays emanating from 'O' appear to emanate from I so as to form perfect image. C is centre of curvature and PCI is the principal axis. Then what is the correct relation between `n_1` and `n_2` ?