This HTML5 app simulates a simple refracting astronomical (inverting) telescope, consisting of two lenses which are called the objective and the eyepiece (ocular). The rays of light incident from the left on the objective are refracted by the objectice and the eyepiece and reach the eye of the person looking through the telescope (to the right of the eyepiece). It is possible to vary the focal lengths of objective and eyepiece from 0.05 m to 0.5 m by using the text fields of the panel. In addition, you can modify the direction of the rays by dragging the mouse. The program will calculate for both objective and eyepiece the angles between the rays of light and the optical axis and the magnification.