Focal Point and Focal Length

Focal Point

Parallel light rays impinge on a concave mirror and reflect. The reflected rays converge in front of the mirror as seen above. The point where the outgoing light paths intersect is called the focal point.

However, parallel light rays reflecting off a convex mirror diverge. In this case the focal point is located behind the mirror where the virtual rays, indicated by the light blue dashed lines, meet.

Focal Length

The focal length is the distance between the focal point and the mirror as shown in the diagrams below.

This is all well and good. But, a truly spherical mirror does not have such a well-defined focal point. In the next section, we take a conceptual look at what is called the "spherical aberration."