A convex lens is placed on an optics bench. The image of an arrowed light source is focused on a piece of translucent glass. The lens can be moved between the light source and glass demonstrating image size dependence on the object and image distances.
Instead of using an image screen, a video camera can be placed at the focal point of the convex lens to collect the image. The video camera ouput is connected to a nearby television monitor. This makes it possible to use the demonstration in a large class..