The dark vanes absorb the rays and the light vanes reflect the rays. Invented by William Crooke in the 1860’s. It demonstrates the conversion of light energy into mechanical energy. The vanes in the evacuated glass bulb are Black and White in color. This is a great way to study how light energy is converted into mechanical energy. The stronger the light, the faster it spins. It measures approx. 5 by 3 inch.