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 We first consider refraction by a single spherical surface and
follow it by thin lenses A thin lens is a transparent optical medium
bounded by two surfaces; at least one of which should be spherical