Crookes radiometer
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.
Inside the bulb, on a low-friction spindle, is a rotor with several (usually four) vertical lightweight vanes spaced equally around the axis.
At these very high vacuums the effect of photon radiation pressure on the vanes can be observed in very sensitive apparatus (see Nichols radiometer), but this is insufficient to cause rotation.
The prefix "radio-" in the title originates from the combining form of Latin radius, a ray: here it refers to electromagnetic radiation.
A Crookes radiometer, consistent with the suffix "-meter" in its title, can provide a quantitative measurement of electromagnetic radiation intensity.
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.
[6] When the radiometer is heated in the absence of a light source, it turns in the forward direction (i.e. black sides trailing).
The first experiment to test this theory was done by Arthur Schuster in 1876, who observed that there was a force on the glass bulb of the Crookes radiometer that was in the opposite direction to the rotation of the vanes.
If light pressure were the cause of the rotation, then the better the vacuum in the bulb, the less air resistance to movement, and the faster the vanes should spin.
The actual pressure exerted by light is far too small to move these vanes, but can be measured with devices such as the Nichols radiometer.
It is in fact possible to make the radiometer spin in the opposite direction by either heating it or putting it in a cold environment (like a freezer) in absence of light, when black sides become cooler than the white ones due to the thermal radiation.
Years after this explanation was dismissed, Albert Einstein showed that the two pressures do not cancel out exactly at the edges of the vanes because of the temperature difference there.
The vanes of a typical Crookes radiometer are not porous, but the space past their edges behaves like the pores in Reynolds's plate.
Because the plates in a radiometer are not fixed, the pressure difference from cooler to hotter side causes the vane to move.
The gas movement causes the light mill to rotate with the concave side moving forward, due to Newton's third law.
This monocolored design promotes the fabrication of micrometer- or nanometer-scaled light mills, as it is difficult to pattern materials of distinct optical properties within a very narrow, three-dimensional space.
[15] 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.
