Why are the blades of a fast moving fan appear to move in opposite direction?
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Well first, I'd like to point out that it's not always in the opposite direction. It could be in the same direction, just much slower than in reality. While a fan is slowing down, you might ‘see’ it reverse direction several times.
The answer lies in persistence of vision - your eye only refreshes the image it sees so many times a second (about 10). It can be helpful to think of your eye as a video camera with a fixed framerate. The spinning in reverse thing is this:
Your eye takes one picture, then the blades of the fan rotate, say 355° clockwise, then your eye takes another picture, so to you it looks like between the first and second picture the fan actually rotated 5° counterclockwise - and this keeps happening between consecutive 'pictures’. Obviously this depends on how much the blades spin in that interval, which depends on how fast they are spinning - which is why it is sometimes backwards and sometimes forwards at all different speeds and sometimes almost stationary.
The answer lies in persistence of vision - your eye only refreshes the image it sees so many times a second (about 10). It can be helpful to think of your eye as a video camera with a fixed framerate. The spinning in reverse thing is this:
Your eye takes one picture, then the blades of the fan rotate, say 355° clockwise, then your eye takes another picture, so to you it looks like between the first and second picture the fan actually rotated 5° counterclockwise - and this keeps happening between consecutive 'pictures’. Obviously this depends on how much the blades spin in that interval, which depends on how fast they are spinning - which is why it is sometimes backwards and sometimes forwards at all different speeds and sometimes almost stationary.
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When a (ceiling) fanrotates fast, why do the blades of the fan appear to slowly rotate in the opposite direction? ... If it moves more than 1/2 rotation per flash it appears to be movingbackwards.
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