How a Radiometer Gets Its Spin
A radiometer’s vanes sit inside a glass bulb with most of the air removed. The remaining gas is thin enough that its molecules can travel relatively far between collisions, yet abundant enough to carry heat and exert pressure. That unusual middle ground is essential: with ordinary air, the effect is overwhelmed by everyday convection; in a perfect vacuum, there would be no gas to drive the rotor.
A Tiny Gap Turns Heat Into Motion
Each vane has a dark face and a shiny face. The dark surface absorbs radiant energy more readily and warms the gas beside it; the shiny surface reflects more energy and leaves its neighboring gas cooler. In the narrow space along a vane’s edge, gas molecules moving from the cooler side toward the warmer side produce a small pressure imbalance. That imbalance pushes the vane, and the forces on all the vanes add up to rotation.
The key is the narrow edge gap, where the gas is thin enough for temperature differences across the vane to affect molecular flow. Light supplies the energy by heating the surfaces, but it does not simply shove the blades like a stream of tiny particles. The rotor’s motion comes from heat acting through the remaining gas.
Why the Toy Needs a Carefully Chosen Vacuum
The bulb is not evacuated completely. Its pressure must be low enough for the edge effect to matter, but high enough for gas molecules to transfer momentum to the vanes. If the bulb contained much more air, warm gas would circulate in broader currents and obscure the delicate mechanism. If it contained almost none, the rotor would have too little gas to push against.
So the radiometer is a small lesson in tuning conditions: a little residual gas, uneven heating, and a gap measured on a tiny scale turn light into motion indirectly. The spinning blades reveal not light pressure, but the behavior of gas where ordinary airflow is squeezed into a narrow space.
