The curve called a parabola has the property that when rays
of light (or heat) parallel to the axis of the parabola are
reflected by the curve, they all pass through a single point.
This point is called the focus of the parabola.
The principle is used the other way round in a car headlight;
the cross-section of a headlight is a parabola, and the bulb
is at the focus. The reflected rays of light then form a parallel