Crooks radometer, Nairobi Industrial Area

The Crookes radiometer, also known as a light mill, is a fascinating scientific instrument that consists of a glass bulb containing a vacuum and a lightweight rotor mounted on a spindle. The rotor features a series of vanes, typically painted black on one side and white or reflective on the other. When exposed to light, particularly sunlight or a strong light source, the vanes spin, with the black side moving away from the light source and the white side moving toward it.

This seemingly paradoxical motion occurs due to the differential heating of the vanes: the black surface absorbs more light energy and heats up more than the white surface, creating a temperature difference that results in a pressure difference of the residual gas molecules within the vacuum. This causes the rotor to spin, demonstrating the conversion of light energy into mechanical energy. The Crookes radiometer serves as an engaging educational tool, illustrating fundamental concepts in physics, such as light absorption, thermal dynamics, and energy transfer.