Ellipse - An Optical Property
Light reflects off a curved surface by forming equal angles (incidence and reflection) with the tangent at the point it falls on the surface.
If we consider the ellipse as a reflecting surface, when a ray of light emitted from one of the foci meets a point on the ellipse, it will reflect off the ellipse and pass through the other focus.
Start the animation in the applet below to view this optical property of the ellipse.
If you Show details, you will see the geometry behind this property: move the point of incidence to verify that the property holds for all points of incidence on the ellipse.