Converging lens
Also called Convex lens
A lens that is thicker at its centre than at its edges and bends parallel rays inward to a real focal point on the far side. Its focal length is positive, and it forms a real image whenever the object lies beyond the focal point.
The CED writes both words every time, "thin convex (converging) lens", because the naming crosses over between lenses and mirrors and the pairing is what keeps it straight.
Positive focal length, because the focal point sits on the transmitted side, where real light actually converges. This is the lens that burns a hole in paper on a sunny day, and the burn mark is the focal point.
Two regimes, split at .
- Object beyond : the image is real, inverted, on the far side. Beyond it is reduced, at the same size, between and enlarged. Camera and projector.
- Object inside : the image is virtual, upright, enlarged, on the same side as the object. This is the magnifying glass.
- Object at : no image forms. , so the rays leave parallel and never meet.
What makes it the opposite of a [diverging lens](/glossary/diverging-lens): a converging lens brings a parallel bundle to a genuine crossing point, so it can produce a real image; a diverging lens spreads the bundle so the rays only appear to come from a focal point on the incident side, and it can never produce one.
Do not carry the word convex over from mirrors. A convex lens converges, while a convex mirror diverges, because one transmits light and the other sends it back. Topic 13.4 has the ray diagrams and the full case table.