Parabola - 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 a parabola as a reflecting surface, when a ray of light emitted from the focus meets a point on the parabola, it will reflect off in a straight line parallel to the axis of the parabola.
The converse also holds, and it's the principle used - for example - to design satellite dishes: any ray parallel to the axis of the parabola will reflect off the parabola and pass through the focus.
Start the animation in the applet below to view this optical property of parabolas.
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 parabola.