Editing Crookes radiometer (section)

===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/>