Page "Crookes radiometer" Paragraph 15

# Crookes incorrectly suggested that the force was due to the pressure of light.

This theory was originally supported by James Clerk Maxwell, who had predicted this force.

This explanation is still often seen in leaflets packaged with the device.

The first experiment to disprove 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.

This showed that the force turning the vanes was generated inside the radiometer.

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.

In 1901, with a better vacuum pump, Pyotr Lebedev showed that in fact, the radiometer only works when there is low pressure gas in the bulb, and the vanes stay motionless in a hard vacuum.

Finally, if light pressure were the motive force, the radiometer would spin in the opposite direction, as the photons on the shiny side being reflected would deposit more momentum than on the black side where the photons are absorbed.

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.