Radiometer
Safety Warnings
Warning: Heat Source Precautions
Purpose
Radiant energy transfer imparts motion: both visible and infrared radiation cause the radiometer vanes to spin. Students see a concrete example of electromagnetic energy converting to mechanical motion and observe that invisible (IR) radiation carries energy just as visible light does.
Figure 1:
Crookes radiometer at rest, showing four vanes with black and reflective sides inside a partially evacuated glass bulb.
Figure 2:
With a light source nearby, the vanes spin as the black surfaces absorb more radiation and heat the residual gas molecules preferentially.