However, we shall restrict our
discussion to the special case of curved surfaces,
that is, spherical surfaces The normal in this case
is to be taken as normal to the tangent to surface
at the point of incidence That is, the normal is
along the radius, the line joining the centre of curvature of the mirror to
the point of incidence We have already studied that the geometric centre of a spherical mirror
is called its pole while that of a spherical lens is called its optical centre