Radiometer

infra-red radiation - kinetic theory of gases - heat energy

What it Shows:
Detection of infra-red radiation by the rotation of a paddle-wheel vane inside a low pressure flask.

How it Works:
A radiometer consists of a partially evacuated flask containing a four bladed vane (see figure 1). One side of each blade is a matt black, the other silver. The black surface, being a better absorber and radiator of heat, warms the air above its surface more than the silver. The resulting higher kinetic energy of these air molecules means that the pressure above the black surfaces is higher; the pressure difference rotates the vane.

The infra-red radiation can be supplied by an IR source^tot, a hair drier (this could give the impression of the vane being blown round!), or a heat lamp.

Figure 1. Schematic diagram of radiometer.

Setting it Up: At a height of 10cm, the radiometer is too small to be seen from the back of a lecture hall. A CCD camera^tot with wide angle lens will blow it up to a nice size.

Comments: Our black and white video cameras^tot are very sensitive to IR. An instructive addition to this demo is to set up the radiometer and IR source so that the source points towards the audience. The IR is completely invisible to the audience, but it behaves as a bright light as far as the video camera is concerned. A reduced pressure inside the flask is essential for the vane to rotate, the pressure difference being so slight. In a full vacuum of course, the effect would be lost. Radiometers are available in a lot of places, including Cenco (77640). Rating **

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