Anyone who's looked at a spinning car wheel or fan notices that sometimes it looks as if the wheel/fan is spinning backwards. I was once told that this is because of the fact that the light shining on it, coming from an electric light source, turns on and off at a certain frequency (~50Hz) and therefore we get a stroboscopic effect. But I see this effect even in daylight - which is constant. What could be the reason for this? Could it have to do with the rate that our eyes can "refresh" the picture sent to the brain? Thanks.