Crookes Radiometer

A Crookes Radiometer, also known as a “light mill,” is a device consisting of an array of vanes mounted on a spindle that can rotate when exposed to light. It was invented by the British scientist Sir William Crookes in 1873.

How It Works:

The radiometer consists of a glass bulb with a partial vacuum inside, within which a spindle is mounted. The spindle holds a set of four black and white vanes, which are attached to it and positioned at angles so they can rotate. The black side of each vane absorbs light, while the white side reflects it. When the bulb is exposed to light, particularly ultraviolet light or intense visible light, the vanes begin to spin.

Explanation of Rotation:

At first, the rotation of the vanes puzzled scientists. A common theory was that the effect was caused by the pressure of light photons (radiation pressure), but the actual cause is somewhat more complex:

• Thermal effect: In a vacuum, the black side of the vane absorbs more light and heats up faster than the white side, which reflects the light. This difference in temperature causes the air molecules near the black side to be heated and push away from it with greater force than on the white side. This imbalance in pressure leads to the rotation of the spindle.

However, the presence of gas molecules in the bulb is key for this effect to happen. In an ideal vacuum, the radiometer wouldn’t spin as expected. Therefore, it suggests a balance between the light and the pressure exerted by gas molecules within the bulb.

Significance:

The Crookes radiometer was an important early experiment in studying the interaction between light and matter. It contributed to the understanding of light’s energy and momentum, though it is now mostly considered a curiosity or demonstration of principles of radiation pressure and thermodynamics.