In geometric optics the calculations for finding out how specular images are formed, are simplified by just considering the rays which are almost parallel to the axis of the mirror or the lens. In the case of a spherical mirror, the position of the image of a point can be calculated using only the intersection of the ray which passes through the centre C and the one which passes through the vertex V, that is, the green and blue rays in the following visual. The other intersections of rays are ignored because we know that they tend to coincide with this one at the limit when the rays are not far from the axis of the mirror.