Today's demonstration from the Cabinet of Physics appears at first to be about optics but, in a surprising twist (SPOILER ALERT), turns out to be about heat.
A curved mirror, a paraboloid, reflects the light of a candle flame to another curved mirror some distance away. The candle sits at the focal point of the first mirror. The second mirror brings the light to its own focal point; hold a white card at this precise spot, and you'll see an image of the candle flame.
Now the candle is replaced by a collection of burning lumps of charcoal. Across the room, a piece of tinder is placed at the focus of the second mirror. To the eye, the coals seem much dimmer than the candle flame. The mirrors are gathering not only the visible light, but also the more copious invisible infrared light emitted by the coals.
The unseen radiant heat is energetic enough to ignite the tinder. Soon it too is glowing red, and smoke is rising. It's a nice example of the close kinship between light and radiant heat.
Perhaps the novelist H. G. Wells had a demonstration like this in mind in 1897 when, in War of the Worlds, he imagined Martian invaders incinerating the English landscape with "heat rays."
This apparatus reminds me of a story. The physicist Theodore Taylor was responsible for a most peculiar application of focused heat radiation. At an observation post in a Nevada desert on 5 June 1952, Taylor was waiting for the test of a nuclear bomb he had designed. He took a small parabolic mirror and attached a rig, made out of wire, that could hold the tip of a cigarette at the mirror's focal point. He aimed the mirror carefully at the distant weapon. At the end of a countdown, intense visible and infrared light engulfed the test site. Miles away, Taylor withdrew his Pall Mall from the mirror and took a puff. He had invented the first nuclear-powered cigarette lighter.
The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.