Reflection at Curved Mirrors
1. Curved and Spherical Mirrors
A curved mirror is a mirror whose reflecting surface is curved (concave, convex or cylindrical). If the reflecting surface is part of a hollow sphere, it is called a spherical mirror. There are two kinds:
- Concave mirror: the reflecting surface curves inward. Parallel rays of light converge to a point after reflection, so it is a converging mirror. It can form a real image, and its focal length is taken as positive.
- Convex mirror: the reflecting surface curves outward. Parallel rays diverge after reflection and appear to come from a point behind the mirror, so it is a diverging mirror. It always forms a virtual image, and its focal length is taken as negative.
2. Terms used for a Spherical Mirror
- Pole (P): the geometric centre of the mirror surface.
- Centre of curvature (C): the centre of the sphere of which the mirror is a part.
- Radius of curvature (R): the radius of that sphere, that is, the distance $CP$.
- Principal axis: the straight line passing through the pole $P$ and the centre of curvature $C$.
- Principal focus (F): the point on the principal axis where rays parallel to the axis actually converge (concave) or appear to diverge from (convex) after reflection.
- Focal length (f): the distance between the pole $P$ and the principal focus $F$.
- Aperture: the effective width of the mirror over which reflection takes place.