The recent celebration of the discoveries made by Galileo four centuries ago has attracted new attention to the refracting telescope and to its use as an instrument for the observation of the night sky. This has offered the opportunity for addressing in the classroom the basic principles explaining the operation of the telescope. When doing so, a key concept that is faced is magnification. In geometrical optics, the treatment of magnification is generally given in terms of light rays and first order (Gaussian or paraxial) ray tracing. Computer programs are available with which the light path through the lenses and the whole telescope can be simulated.