Radiometer – Thermal Transfer

adiometer is a device for measuring the amount of infrared radiation. It consists of freely pivoted rotor with four vanes perpendicular to one another. The vane surface is painted alternate black and silver. The rotor is enclosed in a partial vacuum glass bulb.

In general, when exposed to infrared radiation, the rotor will spin. The greater the amount of radiation, the faster the rotor spins.

The actual working principle is actually much more complicated. Refer to the videos below for detailed explanation.

For our context in O-level, we can briefly explained based on what we learned. As the vanes of the rotor are exposed to infrared radiation, the black side of the vane absorbs more radiation as it is a good absorber of radiation and hence its at higher temperature. The silver side reflects the radiation. The air molecules at the black side will get heated up and gain more kinetic energy. Hence rate of collision is higher and the air molecules collide on the black side with more force than the silver side. This results in a net force on the black surface and the rotor spins in a specific direction as shown in the video.

The more complicated theory how radiometer works