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focal point

Each (rotation symmetric) lens has two focal points on it’s optical axis.
They’re located where images of infinite distanct objects are generated.
The focal points belong to the Gauss-points.

When a ray of light is sent parallel to the optical axis into a lens or lens system, then the ray or it’s prolongation intersects the optical axis after exiting the last lens.

This intersection with the optical axis is called focal point.

The name is derived from “burning glasses” (imagine a magnifying glass) with which the (nearly parallel) sun beams are bundled to one point.
At this point it gets so hot that wood or paper placed at this spot starts to burn.