Focus of a Parabola
The focus of a parabola is a fixed point on
the interior of a parabola used
in the formal definition of the curve.
A parabola is defined as follows: For a given point,
called the focus, and a given line not through the focus, called
the directrix, a parabola is the locus of points such that the
distance to the focus equals the distance
to the directrix.
For a parabolic mirror, all rays of light emitting from the focus
reflect off the parabola and travel parallel to each other (parallel
of symmetry as well).
Conic sections, foci
of an ellipse, foci of a hyperbola,