An optical property of the parabola:
rays emanating from the parabola focus travel, after reflection in the parabola, parallel to the principal axis of parabola;
rays traveling parallel to the principal axis of parabola are directed, after reflection in the parabola, at the focus of parabola.
The optical property of a parabola can be illustrated by releasing a ball from your hand over a lying parabolic dish (e.g. from a small satellite dish) with the ball fixed in focus.
The photons of light are the peculiar balls from the previous experiment. Even a small metal parabolic mirror pointed at the Sun gathers enough energy in focus to ignite paper.
The experiment requires two paraboloids on the same axis inside the room. Lighting a light bulb in the centre of one paraboloid will cause the paper to ignite at the focus of the other paraboloid. More surprisingly, if a piece of ice is placed at the focus of one paraboloid and a thermometer is placed at the focus of the other, the thermometer reading will be below room temperature.
The optical property of the parabola is also used for cooking outdoors. It's worth trying this fascinating experiment with children at school, but both in this and previous experiments, you have to be very careful because the temperature in focus is very high..
