1. Black and White Vanes:
- The vanes of a radiometer are typically painted black on one side and white on the other.
2. Light Absorption and Heat:
- The black side absorbs more light energy than the white side. This absorbed energy causes the black side to heat up more than the white side.
3. Gas Molecules:
- Inside the radiometer is a partial vacuum containing a small amount of gas.
4. Molecular Collisions:
- The heated black side of the vane transfers its heat to the surrounding gas molecules. These molecules collide with the vane, transferring momentum and pushing it away from the hotter side.
5. Recoil Effect:
- Since the molecules are moving away from the hotter side, the vane experiences a net force in the opposite direction (towards the cooler side).
6. Rotation:
- The vanes are mounted on a central spindle, and the force pushing them towards the cooler side causes them to spin.
Essentially: The light energy absorbed by the black side of the vanes heats the surrounding gas molecules, causing them to collide more vigorously with the black side. This results in a net force that pushes the vanes away from the black side, making them spin.
Important Note: The radiometer does not spin due to the direct pressure of sunlight on the vanes. The pressure of sunlight is too weak to cause significant rotation. The spinning effect is primarily due to the interaction between the heated vanes and the gas molecules inside the radiometer.
