This interactive sketch produces the image formed by a converging lens as the user moves the object. It requires Geometer's Sketchpad to run. The sketch shows classic ray diagrams and calculations for an image formed by an object placed anywhere in front of or behind a converging lens. It also illustrates a three dimensional figure and focusing an image on a screen. It can be used as a dynamic teacher illustration or as an interactive student exploration. You must have Geometer's Sketchpad to run this illustration.