MOTION MOUNTAIN

light sources 145
Figure to be inserted
Challenge 156 s
F I G U R E 91 Bodies inside an oven at room temperature differ in colour, in contrast to bodies at high
temperature (photo © Wolfgang Rueckner).
F I G U R E 92 The last mirror of the solar furnace at
Odeillo, in the French Pyrenees (© Gerhard
Weinrebe).
contrast nor brightness changes exist that allow one to distinguish the objects from the
walls or their surroundings. Can you explain the finding?
In short, we are able to see each other only because the light sources we use are at
a different temperature from us. We can see each other only because we do not live in
thermal equilibrium with our environment.
Limits to the concentration of light
Light sources should be as bright as possible. Are there any limits? Interestingly, for black
body radiation there is an important and instructive limitation.
If we build a large lens or a large curved mirror, we can collect the light of the Sun
and focus it on a tiny spot. Everybody has used a converging lens as a child to burn black
spots on newspapers – or ants – in this way. In Odeillo, in Spain, wealthier researchers
have built a curved mirror as large as a house, in order to study solar energy use and
material behaviour at high temperature. Essentially, the mirror provides a cheap way to
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net