Crookes Radiometer

Crookes radiometer.

A radiometer running.

The Crookes radiometer or light-mill (light-mill) is a device invented in 1873 by the English chemist William Crookes. It consists of four arms that each hold a blade or plate at its ends, painted white on one side and black on the other. The four arms that support the plates are suspended from a needle and supported by a glass shaft to reduce friction as much as possible. This mill is inside a sealed glass sphere and in which a partial vacuum has been made.

The blades rotate when exposed to light, the rotation being faster the more intense the incident light is. That provides a quantitative measure of the intensity of electromagnetic radiation. The explanation for the rotation of this device has historically been the reason for much scientific controversy.

Crookes got the idea from some chemical research he was doing. In the course of chemical experiments that required very precise quantitative measurements, he was weighing samples in a chamber under partial vacuum, in order to reduce the effect of air currents. Suddenly, he noticed that the value of the weights was disturbed when sunlight fell on the balance. Investigating that effect, he created the device named after him. Crookes radiometers are still manufactured and sold for recreational or educational purposes.

Operation

The moment of force generated by the plate system is very small, since both the length of the arm and the mass of the plate are very small, so the axis must be very well balanced and must have practically no friction. null so it can rotate.

Crookes wanted to know if light hitting a surface exerted any force, so he thought light would bounce off the silver sides of the plates, while being absorbed by the black side. If all there was was a pure momentum transfer between the incident photons and the plates, we would have the plates rotate so that the black side was in front, since absorbing the photons there would take less momentum than on the silver sides, where the photons are reflected (bounced). But Crookes was surprised when he observed that his radiometer rotated in the opposite way to what was expected (the black side of the plates moved away from the light).

Originally it was thought that the rotation was produced by the heating of the black sides of the plates, but in later experiments it was found that the radiometer rotated in the opposite direction again (black side going towards the light) if it cooled suddenly. This contradicted that original hypothesis and many other theories) since the light side could not heat up and thus produce the spin.

The explanation was found by two great scientists, James Clerk Maxwell and Osborne Reynolds: the real effect occurs at the edges of the vanes.

Basically, on the hot side, the gas molecules are moving with a higher average velocity than the gases on the cold side. When hot molecules hit the edge of the paddle, on average they will produce a force on the paddle that is toward the cool side. Since the average velocity of the hot molecules is greater than the average velocity of the cold molecules, there will be a force on the paddle toward the cool side. This effect was called 'thermal drag'.

In later, more advanced experiments and with an almost perfect vacuum, it was determined that light does exert a force.^{[citation needed]}