Concave mirror
Also called Converging mirror
A mirror curved so that its reflecting surface caves inward, bringing parallel rays toward a real focal point in front of it. Its focal length is positive and equal in size to half the radius of curvature.
A converging mirror, and the CED writes it as "concave (converging) mirror" every time so the shape word never travels alone.
Positive focal length, because the focal point sits in front of the mirror, on the side real light occupies. The CED says that point may be approximated as halfway between the surface and the centre of curvature, so for rays near the axis. That relation is not printed on the equation sheet: halve the radius yourself.
The only optic in Unit 13 with a full story. As the object walks in toward the mirror:
| Object position | Image |
|---|---|
| Beyond | Real, inverted, reduced |
| At | Real, inverted, same size |
| Between and | Real, inverted, enlarged |
| At | No image, reflected rays leave parallel |
| Inside | Virtual, upright, enlarged |
The switch happens at the focal point, and at that exact position , so the image distance runs to infinity. Put a bulb there instead and you get a beam, which is a headlamp.
What makes it the opposite of a [convex mirror](/glossary/convex-mirror): it gathers a parallel bundle to a real crossing point, so it can throw a real image onto a screen and can concentrate sunlight. A convex mirror never can. The shaving mirror and the makeup mirror are the last row of the table, used deliberately inside .