Recoiling Photoelectrons

In my contribution to CNPS 2016 “Physics without Paradoxes” I have made a statement that the effect in Crooke’s radiometer depends on a recoil effect from photoelectrons emitted from the black (most absorbing) side of the radiometer. However, an alternative interpretation has been suggested. Since I have problems accepting this alternative I will explain my motivations in more detail.

The alternative interpretation states that air (of very low pressure) has higher temperature in front of the black surface. This means that temperature rotates with the same frequency as the radiometer. However, I have 2 arguments against this interpretation. The first argument is that the temperature difference between 2 sides of a thin metal is small and also that both sides have almost the same emission factor at 300 K (or 10 mikro-meters). Therefore, thermal emission is almost symmetric. The second argument is the extremely good transmission of 300 K radiation in air. On a clear night the temperature is falling rapidly due to the good transmission through many kilometers in normal atmosphere. The transmission through a few centimeters of extremely low pressure air would be very good. These 2 arguments together means that temperature changes rotating inside the radiometer are not very plausible and this interpretation therefore is unrealistic.

I conclude therefore that Crooke’s radiometer is best explained by recoil from photoelectrons.

John-Erik Persson