Virtual images from convex and concave lenses

When rays of light are brought to a focus a real image forms. A real image can be projected onto a screen. Virtual images occur at points where rays appear to (but do not actually) originate from. They cannot form on a screen. The size and position of a virtual image is found by drawing virtual rays to find where rays appear to originate from. Upright virtual images are produced by concave lenses, and objects within a focal length of a convex lens. Convex lenses fix long sight by shortening the focus. Concave lenses fix short sight by lengthening the focus.