Also known as the light mill or solar engine, the solar radiometer demonstrates with simple physics and movements how light can be transformed into energy. Its invention in 1873 is credited to Sir Williams Crookes, an English physicist. The black polished vanes are suspended in the semi-vacuum created inside the vacuum created inside the radiometer. When exposed to a warm light source, such as the sun or a lamp, the vane spins. The more powerful the light source the faster the vanes will spin. The black side of the vane absorbs the light faster than the temperature of the air molecules on the back side of the vane causing the spinning motion.