**Foci of an Ellipse**

Two fixedpoints on the interior of an ellipse used in the formal definition of the curve. An ellipse is defined as follows: For two given points, the foci, an ellipse is the locus of points such that the sum of the distance to each focus is constant.

Note: If the interior of an ellipse is a mirror, all rays of light emitting from one focus reflect off the inside and pass through the other focus.

**See also**

Directrices of an ellipse, foci of a hyperbola, focus of a parabola, focal radius

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