A lens forms an image by refracting light. In a convex lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length. Ray diagrams are used to show the formation of images by convex and concave lenses. • The image produced by a convex lens can be either real or virtual. The image produced by a concave lens is always virtual. • Students should be able to construct ray diagrams to illustrate the similarities and differences between convex and concave lenses. • In ray diagrams a convex lens will be represented by [a vertical line with outward-pointing arrowheads at each end] and a concave lens will be represented by [a vertical line with inward-pointing arrowheads at each end].