# foci of an ellipse

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.