5 REFRACTION AT SPHERICAL SURFACES
AND BY LENSES
We have so far considered refraction at a plane interface We shall now
consider refraction at a spherical interface between two transparent media An infinitesimal part of a spherical surface can be regarded as planar
and the same laws of refraction can be applied at every point on the
surface Just as for reflection by a spherical mirror, the normal at the
point of incidence is perpendicular to the tangent plane to the spherical
surface at that point and, therefore, passes through its centre of
curvature