Crookes Radiometer

Quick Answer: The Crookes Radiometer, or light mill, is a sealed glass bulb holding a partial vacuum and a set of vanes on a needle pivot, each vane dark on one side and light or silvered on the other. In sunlight or lamplight the vanes spin, turning absorbed radiation into visible motion.

What Makes the Vanes Turn

A Crookes radiometer needs no batteries or wires: set it in a sunny window and the vanes begin to turn, faster as the light gets brighter. The dark faces absorb more radiation than the light faces and become slightly warmer. Gas molecules left in the bulb bounce off the warmer faces with a little more energy and, more subtly, gas creeps around the vane edges from the cool side to the warm side, an effect called thermal transpiration. The net result is that the dark faces move away from the light, so the vanes turn with the light faces leading.

Sir William Crookes built the device in the 1870s while working with vacuum tubes, and at first many people believed it was driven by the pressure of light itself. That cannot be right: light pressure would push harder on the reflective faces and turn the vanes the other way. The accepted explanation, worked out over the following years by Osborne Reynolds, James Clerk Maxwell and others, depends on the few molecules remaining in the bulb, and a radiometer with a very hard vacuum hardly turns at all.

That history makes the light mill useful for teaching how scientific explanations are tested and corrected, as well as radiation and absorption. It also responds to heat: warm the bulb near a lamp and it turns, then move it somewhere cold and it may briefly spin backwards as the dark faces lose heat faster. Bulb size and stand design should be confirmed at enquiry.

Specifications

Classroom Uses

• Comparing how dark and light surfaces absorb radiation
• Showing that the rotation speed rises with light intensity
• Discussing the rejected light-pressure explanation and how it was disproved
• Linking the effect to gas molecules and kinetic theory in older classes

Care & Handling

• Handle the Crookes radiometer by its base only; the thin glass bulb breaks easily and the vacuum is lost if it cracks.
• Do not leave it on a hot radiator or close to a heat lamp for long periods.
• Store it upright in its box so the vanes stay seated on the pivot.
• Clean the outside of the bulb with a soft cloth and glass cleaner only.

Why Choose LabEquip

Science teachers value the radiometer because it rewards careful thinking: the obvious explanation is wrong, and working out the right one teaches a lot. LabEquip lists it in the STEM kits range with the Heat Absorption Demonstration Model, which compares black and white surfaces in a more direct way. Ask for quantities through the contact page.

Frequently Asked Questions

Which way do the vanes turn?

The dark faces move away from the light, so the vanes rotate with their light or shiny faces leading. This is the opposite of what light pressure alone would cause, which is the key clue to the real explanation.

Is the radiometer driven by the pressure of light?

No. Light does exert a tiny pressure, but it would push more on the reflective faces and turn the vanes the other way. The real cause is the behaviour of the thin gas left in the bulb near the unevenly heated vanes.

Why does it need a partial vacuum?

At normal air pressure, air resistance stops the vanes and the thermal effects are too small. In a near-perfect vacuum there are too few molecules to push on the vanes. The light mill works in between, at low pressure with some gas remaining.

Does it work under classroom lights?

Ordinary room lighting may turn it only slowly or not at all. Direct sunlight or a filament lamp close to the bulb gives the strongest rotation, because they deliver more of the infrared radiation that warms the dark faces.

Can it spin backwards?

Yes. If a warmed radiometer is moved somewhere cold, the dark faces lose heat faster than the light ones and become cooler. The gas effects reverse and the vanes can turn briefly in the opposite direction.

How fragile is the radiometer?

The bulb is thin glass, so it must be handled gently by the base and kept away from the edge of the bench. If it breaks, the vacuum is lost and the device will no longer work.

Last Updated: September 2026