4D20.10 • Mean Free Path: Crookes’s Radiometer

Photo of the Demonstration at the University of Texas at Austin.

Instructions / Notes:

• A Crooke's radiometer is placed in front of an electric lamp - The lamp is as a source of radiant heat energy.
• The interior of the Crooke's radiometer is under partial vacuum.
• One side of each pane is painted white, while the other side is black.
• Thus, there is a small heat differential between the two surfaces.
• A second radiometer, with a mirrored side, is used to show what happens when less radiant energy strike the panes.

Last updated on June 22, 2026