Radiometer Crooke's Single

Sir William Crookes invented the Radiometer Crooke's Single, a scientific device that accurately measures radiant flux, or the amount of radiant energy. It features a set of vanes or blades within a partial vacuum, distinguishing it from other radiometers. 

The Radiometer is a means of showing the relation between heat and the molecular activity of a gas in a visible way.  It also shows that light is a form of energy and that it can be converted into other forms of energy.  The vanes of Radiometer Crooke's Single exhibit a reaction to light or radiant energy, as they are subject to radiation pressure. While the reflective side reflects light, the absorptive side absorbs it.