15 shows the geometry of formation of image I of an object O on
the principal axis of a spherical surface with centre of curvature C, and
radius of curvature R The rays are incident from a medium of refractive
index n1, to another of refractive index n 2 As before, we take the aperture
(or the lateral size) of the surface to be small compared to other distances
involved, so that small angle approximation can be made In particular,
NM will be taken to be nearly equal to the length of the perpendicular
from the point N on the principal axis