Han, Li-Hsin; Shaomin Wu; J. Christopher Condit; Nate J. Kemp; Thomas E. Milner; Marc D. Feldman; Shaochen Chen (2010). "Light-Powered Micromotor Driven by Geometry-Assisted, Asymmetric Photon-heating and Subsequent Gas Convection". Applied Physics Letters. 96 (21): 213509(1–3). Bibcode: 2010ApPhL..96u3509H. doi: 10.1063/1.3431741. Archived from the original on 22 July 2011. Calaprice, Alice; etal. (27 October 2015). An Einstein encyclopedia. Princeton University Press. p.190. ISBN 978-0691141749. Maxwell, J. Clerk (1 January 1879). "On stresses in rarefied gases arising from inequalities of temperature". Philosophical Transactions of the Royal Society of London. 170: 231–256. doi: 10.1098/rstl.1879.0067.

The air pressure inside the bulb needs to strike a balance between too low and too high. A strong vacuum inside the bulb does not permit motion, because there are not enough air molecules to cause the air currents that propel the vanes and transfer heat to the outside before both sides of each vane reach thermal equilibrium by heat conduction through the vane material. High inside pressure inhibits motion because the temperature differences are not enough to push the vanes through the higher concentration of air: there is too much air resistance for "eddy currents" to occur, and any slight air movement caused by the temperature difference is damped by the higher pressure before the currents can "wrap around" to the other side. [6] Movement with radiation [ edit ] The currently accepted theory was formulated by Osborne Reynolds, who theorized that thermal transpiration was the cause of the motion. [11] Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behaves like the pores in Reynolds's plate. As gas moves from the cooler to the hotter side, the pressure on the hotter side increases. When the plate is fixed, the pressure on the hotter side increases until the ratio of pressures between the sides equals the square root of the ratio of absolute temperatures. Because the plates in a radiometer are not fixed, the pressure difference from cooler to hotter side causes the vane to move. The cooler (white) side moves forward, pushed by the higher pressure behind it. From a molecular point of view, the vane moves due to the tangential force of the rarefied gas colliding differently with the edges of the vane between the hot and cold sides. [3] Brush, S. G.; Everitt, C. W. F. (1969). "Maxwell, Osborne Reynolds, and the Radiometer". Historical Studies in the Physical Sciences. 1: 105–125. doi: 10.2307/27757296. JSTOR 27757296. Han, Li-Hsin; Shaomin Wu; J. Christopher Condit; Nate J. Kemp; Thomas E. Milner; Marc D. Feldman; Shaochen Chen (2011). "Light-Powered Micromotor: Design, Fabrication, and Mathematical Modeling". Journal of Microelectromechanical Systems. 20 (2): 487–496. doi: 10.1109/JMEMS.2011.2105249. S2CID 11055498.

The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity.

In 2010 researchers at the University of California, Berkeley succeeded in building a nanoscale light mill that works on an entirely different principle to the Crookes radiometer. A gold light mill, only 100 nanometers in diameter, was built and illuminated by laser light that had been tuned. The possibility of doing this had been suggested by the Princeton physicist Richard Beth in 1936. The torque was greatly enhanced by the resonant coupling of the incident light to plasmonic waves in the gold structure. [16] See also [ edit ] Thermodynamic explanation [ edit ] A Crookes radiometer in action with the light switched on and off. (Note that the explanation given in the caption to the clip doesn't agree with the modern explanation.) Movement with absorption [ edit ]Loeb, Leonard B. (1934) The Kinetic Theory of Gases (2nd Edition);McGraw-Hill Book Company; pp 353–386 Radiometers are now commonly sold worldwide as a novelty ornament; needing no batteries, but only light to get the vanes to turn. They come in various forms, such as the one pictured, and are often used in science museums to illustrate "radiation pressure" – a scientific principle that they do not in fact demonstrate.