2006-08-10 03:27:10 · 5 answers · asked by abhi s 1 in Science & Mathematics ➔ Physics
Sampling of information must be done at twice the periodic speed of the phenomena being sampled.
Our eyes have a certain sampling speed of acquiring and correlating information received this is why when something is moving past you gets blurred up. In case of periodic motion such as a fan our visual processor, by under sampling, introduces an error which gets interpreted as a one direction of rotation and being compared to the earlier memory when the fan was rotating slower.
2006-08-10 03:33:51 · answer #1 · answered by Edward 7 · 1⤊ 0⤋
The public power supply is AC. The frequency is either 60Hz or 50Hz depending upon where you live in the world (60Hz in the US).
The motors used to drive fans run a a speed that is a multiple of 50/60 rpm. The drag caused by the fan blades bearings etc generally results in the speed being a little slower.
Strip lighting, although appearing to be uniform in brightness to the human eye, does in fact vary in intensity at a rate of 50/60 Hz and acts like a strobe light.
The strobe effect on fan blades rotating at a multiple of the strobe rate will make the blades appear to stand still. Because drag is causing the blades rotation to be slowed, the fan blades appear to be rotating backwards.
2006-08-10 10:41:35 · answer #2 · answered by Stewart H 4 · 1⤊ 1⤋
It has to do with how your vision works. Your eyes and brain can only process so many images per second. When a fan is moving slowly, there is only a slight turn between each successive image, so it looks like the fan is moving forward. But when the fan is moving faster, the blade goes farther in between images. Instead of being just in front of the previous image, it will appear to be just behind the position of the fan blade in front of it. Since all the fan blades are identical, your brain interprets these mental images to mean that the fan is going backwards.
At even faster speeds, the fan will appear to move forward again. In fact, if you watch a fan starting up, it will appear to go forward, back, forward, back, as long as it is accelerating.
Stewart H also has a point with the strobe-effect in lighting. This is common with indoor fans. However, the effect I described can also be seen in natural sunlight.
2006-08-10 10:43:00 · answer #3 · answered by J C 3 · 2⤊ 1⤋
It's the same reason sometimes wheels on moving cars seem to stand still or go backwards,it's because the frame rate of the camera matches the RPM of the wheel or fan or whatever.
2006-08-10 10:38:34 · answer #4 · answered by booboo 7 · 1⤊ 0⤋
It is an optical illusion.
2006-08-10 10:39:00 · answer #5 · answered by Anonymous · 0⤊ 0⤋