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Focal Point and Focal Length

Incoming parallel rays convene at the focal point.

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.

The focal point of a convex mirror is behind the mirror.

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.

Focal length for a concave mirror. Focal length for a convex mirror.

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."

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