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Focus

Mirrors The focus of a lens or parabolic mirror is the point onto which collimated light parallel to the axis is focused. The focus of an elliptic mirror is either of two points such that light from one converges on the other. The focus of a hyperbolic mirror is either of two points such that light from one is reflected as if it came from the other.

Geometry In an ellipse, foci are the two "off-center" points.

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