# Parabolic Mirror, Theory

*Parabola* is a conic defined by its *focal property*: there is a point - focus - and a line - directrix - and parabola is the locus of points equidistant from the focus and the directrix.

It is known that the tangent to parabola at a point bisects the angle between the segments joining the point to the focus and the directrix.

This is equivalent to saying that, if a light source is located at the focus, the rays from the source reflected in the inner surface of the parabolic mirror are parallel. What good is this property? Well, this is how modern car lights work. And this is the principle underlying radio telescopy.

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