Editing Crookes radiometer (section)

==Thermodynamic explanation==
[[File:24. Круксов радиометар.ogv|thumb|280px|right|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===
When a [[radiant energy]] source is directed at a Crookes radiometer, the radiometer becomes a heat engine.<ref name=Kraftmakher>{{cite book |last1=Kraftmakher |first1=Yaakov |title=Experiments and demonstrations in physics |date=29 August 2014 |publisher=World Scientific |location=Singapore |isbn=9789814434904 |page=179 |edition=2}}</ref> The operation of a heat engine is based on a difference in [[temperature]] that is converted to a mechanical output. In this case, the black side of the vane becomes hotter than the other side, as radiant energy from a light source warms the black side by [[Absorption (electromagnetic radiation)|absorption]] faster than the silver or white side. The internal air [[molecule]]s are heated up when they touch the black side of the vane. The warmer side of the vane is [[#Explanations for the force on the vanes|subjected to a force]] which moves it forward.

The internal temperature rises as the black vanes impart heat to the air molecules, but the molecules are cooled again when they touch the bulb's glass surface, which is at ambient temperature. This heat loss through the glass keeps the internal bulb temperature steady with the result that the two sides of the vanes develop a temperature difference. The white or silver side of the vanes are slightly warmer than the internal air temperature but cooler than the black side, as some heat conducts through the vane from the black side. The two sides of each vane must be thermally insulated to some degree so that the polished or white side does not immediately reach the temperature of the black side. If the vanes are made of metal, then the black or white paint can be the insulation. The glass stays much closer to ambient temperature than the temperature reached by the black side of the vanes. The external air helps conduct heat away from the glass.<ref name=Kraftmakher/>

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.<ref name=Kraftmakher/>

===Movement with radiation===
When the radiometer is heated in the absence of a light source, it turns in the forward direction (i.e. black sides trailing). If a person's hands are placed around the glass without touching it, the vanes will turn slowly or not at all, but if the glass is touched to warm it quickly, they will turn more noticeably. Directly heated glass gives off enough infrared radiation to turn the vanes, but glass blocks much of the far-infrared radiation from a source of warmth not in contact with it. However, near-infrared and visible light more easily penetrate the glass.

If the glass is cooled quickly in the absence of a strong light source by putting ice on the glass or placing it in the freezer with the door almost closed, it turns backwards (i.e. the silver sides trail). This demonstrates radiation from the black sides of the vanes rather than absorption. The wheel turns backwards because the net exchange of heat between the black sides and the environment initially cools the black sides faster than the white sides. Upon reaching equilibrium, typically after a minute or two, reverse rotation ceases. This contrasts with sunlight, with which forward rotation can be maintained all day.