PURPOSE: Show that radiation from a cold object is cold.

DESCRIPTION: If you put a hot object at the focus of one of the concave parabolic mirrors and a thermal probe at the focus of the other mirror, heat from the hot object will heat up the probe, yielding a temperature rise of the thermometer. (Compare the left and center pictures above.) If you put something very cold at the first focus, the temperature will drop. (Compare the left and right pictures above.) This demands a rather different explanation - blackbody radiation emitted by all objects - than the rather simple explanation given in the case of the hot object.

The photographs below include hot thermal probe (left), the hot match flame (center) and the cold copper cylinder dipped in liquid nitrogen (right).

This experiment demands the proper explanation in terms of blackbody radiation emitted by all objects, not just "hot" objects. The historical struggle of physicists to deal with this is documented in an interesting article by Hasok Chang, Lecturer in Philosophy of Science at University College, University of London, entitled Rumford and the Reflection of Radiant Cold: Historical Reflections and Metaphysical Reflexes, in Physics in Perspective Volume 4 Issue 2 (2002), pp 127-169. Click here for the abstract of this article.

SUGGESTIONS:

See Question of the Week #148 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Parabolic reflectors accessories from demonstration L3-16: FOCUSING OF HEAT WAVES BY MIRRORS, thermal probe with display unit, copper cylinder from demonstration I5-51: SPECIFIC HEAT - ALUMINUM AND COPPER, supports for thermal probe and copper cylinder.

SETUP TIME: 10 minutes.