The radiation pressure of direct sunlight is about 5 x N/. The effect of even less radiation can be shown with an apparatus where disks are placed at the ends of bars that are connected to a bearing. If one side of each disk is black and the other side is a mirror, then the light striking the black side is completely absorbed, so all the momentum of the light is transferred to the disk, while the light incident on the mirror is almost all reflected. Therefore the momentum transferred to the mirror is about twice as great as that transferred to the disk facing the light with its black side. The difference in momentum causes the rotor to rotate. It is important to ensure low friction at the bearing for the apparatus to work. In order to eliminate the effects of thermal expansion and air friction, the apparatus must be under high vacuum.