Convex Mirrors

A convex mirror curves outward toward the object, like the back of a spoon. Parallel rays hitting that surface reflect outward too, spreading apart instead of meeting. Because the rays diverge, they never cross in front of the mirror. Your brain, which assumes light travels in straight lines, traces the diverging rays backward and sees them appear to come from a point behind the mirror's surface. That point is the image: virtual, upright, and smaller than the object, sitting between the mirror and its focal point.

The ray diagram uses the same two easy rays as a concave mirror, just with reflections that bend away from the axis. A ray parallel to the axis reflects as if it came from the focal point behind the mirror. A ray aimed at the focal point in front reflects back parallel to the axis. Extend both reflected rays backward behind the mirror and they meet at the image location.

The mirror equation 1/do + 1/di = 1/f still works, but a convex mirror always carries a negative focal length, since its focal point sits behind the mirror. That sign forces di to come out negative too, which means virtual, every time, for every object distance. This is the key contrast with concave mirrors from the previous topic, where the image can be real or virtual depending on where the object sits. Convex mirrors never have that choice, which is exactly why they are used for wide, safe fields of view in car mirrors and store security domes.