Parabola: optical property

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..

Other models in “Conic sections: parabola”