Radiometer
A radiometer is a detector used to measure irradiance . However, the term radiometer is not used for just one principle of operation.
The light mill invented by William Crookes - a rotating impeller when illuminated - does not measure the mechanical radiation pressure of the light, but is driven by thermal molecular movement. The interior of the glass bulb is largely evacuated.
Function of a light mill
The gas molecules in the radiometer constantly collide with the wings of the radiometer. When a particle on the surface and a gas molecule collide, the direction in which momentum and energy are transferred depends on their energy.
If the surface of a wing is warmer than the gas, an impacting gas particle will absorb part of the momentum from a particle on the wing surface. The recoil that occurs drives gas particles and wings apart. If the wings are evenly colored, the impulses of the particles that collide from all sides cancel each other out. The wing will not move then.
If one side is warmer than the other, the momentum that the impacting gas particle takes with it when it bounces back is greater on the warmer side than on the colder side. This temperature difference is reached when one side absorbs the radiation more than the other. With visible light, a black wing side is usually combined with a white or reflective wing surface.
With low-friction storage and enough light, the radiometer starts moving.
The effect shows the following peculiarities:
- The dark side, which absorbs the radiation more strongly, turns away from the observer.
- The radiometer also rotates with diffuse radiation.
- The best performance is shown when the gas pressure in the radiometer is in the range of the fine vacuum , here around one to ten Pascal .
The radiometer force increases as the number of particles increases, but only as long as the free path of the gas molecules is large compared to the size of the radiometer. If the number of particles increases further, they can no longer carry the momentum far enough away from the wing due to the shortening path length.
The effect is referred to in the specialist literature as the "radiometer effect".
Other devices
In contrast to the radiometer, a similar, approximately simultaneous, but far more sensitive setup by Ernest Fox Nichols actually measures the mechanical radiation pressure of light directly . This also applies to an experiment developed by Pyotr Nikolayevich Lebedev around this time . In these experiments, the space around the impeller must be evacuated to a much greater extent and the mechanical suspension must be carried out with very little friction.
literature
- ↑ a b Klaus Lüders, Robert O. Pohl (ed.): Pohl's introduction to physics . Volume 1: Mechanics, acoustics and thermodynamics. 21st edition. tape 2 . Springer-Verlag, Berlin Heidelberg 2017, ISBN 978-3-662-48662-7 , 16.2 Recoil of gas molecules during reflection, radiometer force.
- ↑ Harry Paul (ed.): Lexicon of optics in two volumes, spectrum Akademischer Verlag, Heidelberg, entry Radiometer .
