1  Refraction at a spherical surface
Figure 9 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