In the paraxial approximation, i.e. in the limit of small angles to the optical axis, the rays emitted by a source and reflected in a spherical mirror, or their extensions, cross at points (the real or virtual images of the source) that appear as new sources to an observer. Rays forming greater angles to the optical axis no longer meet exactly at one point, a phenomenon known as spherical aberration, reducing the quality of the image produced by the mirror. This demonstration shows how the image is formed, also in relation to the dispersion of the rays with respect to the optical axis.