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The vacuum is important to the radiometer's success. If there is no vacuum (that is if the bulb is full of air) the vanes do not spin because there is too much drag. If there is a nearperfect vacuum the vanes do not spin unless they are held in a frictionless way. If the vanes have a frictionless support and the vacuum is complete then photons bouncing off the silver side of the vanes push the vanes causing them to rotate. However this force is exceedingly small. If there is a good but incomplete vacuum then a different effect called thermal transpiration occurs along the edges of the vanes as described on this page. The effect looks as though the light is pushing against the black faces. The black side of the vane moves away from the light. This is the case with this radiometer.