# Confocal

In geometry, **confocal** means having the same foci: confocal conic sections.

- For an optical cavity consisting of two mirrors, confocal means that they share their foci. If they are identical mirrors, their radius of curvature,
*R*_{mirror}, equals*L*, where*L*is the distance between the mirrors. - In conic sections, it is said of two ellipses, two hyperbolas, or an ellipse and a hyperbola which share both foci with each other. If an ellipse and a hyperbola are confocal, they are perpendicular to each other.
- In optics, it means that one focus or image point of one lens is the same as one focus of the next lens.

## See also

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