Can anybody explain me about the visual effect of backward rotation of wheels of the vehicles seen in movies?

Question

I want answers based on SAMPLING THEOREM only.

Answer 1

Well, in movies, you typically are getting 30 frames per second or samples per second, basically a snapshot. Or one such snapshot every 0.033 seconds. So, in the first snapshot, let's suppose that a spoke of the wheel is pointing directly up. In the next picture, suppose the wheel has rotated such that the nearest spoke to being straight up and down is 1/2" behind the vertical. Then in the next spanshot the spoke is 1" behind the vertical. Therefore, the wheel will appear to be moving in reverse. If the car speeds up slightly, the wheel will look as if it is not spinning at all and a little faster will make it appear that it is spinning forward again.

The same thing will occur in real-life as well because your brain and eyes will try to make sense of what you are seeing as well. If you are beside a car with spokes, try blinking your eyes really fast and deliberate. You can create a similar illusion as you get on the movies if the other car isn't going too fast. You can actually read the writing on the tire itself by doing this, again if the other isn't moving too fast. BTW, try to not do this while you are driving, that could be dangerous.

Answer 2

Anytime you take a "moving picture" of some event your equipment is actually taking a series of "instantly frozen" still pictures. When you visually watch such a "series", your eyes relay the "still" images to your brain which perceives the event to be in motion as it was when you recorded it. If something in the images is moving cyclicly, such as a rotating wheel, then each "still" image catches the wheel rotated to a slightly different position from the previous "still" image. If the rotational speed of the wheel (say at 0.98 revolution per minute--for example) is just slightly slower than the frequency at which you are taking the "still" pictures (say at one image per minute) then in each "still" image (called a "frame" in a camera) the wheel will appear to be rotated less and less and your brain will register it as turning backwards instead of foreward! If the rotational speed of the wheel is exactly the same as the frequency at which you are recording your images, your brain will think it is not turning at all! If the wheel is rotating just slightly faster (say 1.02 revolution each minute) your brain will perceive it to be turning "foreward" but very slowly! In the "real" world wheels and cameras (or video cameras) don't operate at 1 revolution or 1 image per minute, HOWEVER-- as long as the rotational speed of the wheel is some exact multiple of the recording speed of your camera, the effect on your brain will be the same as the 1 to 1 relationship I described above! Example-- your video camera takes 60 images per minute--(1 per second)--If your wheel is rotating twice as fast (120 revs. per minute--rpms) or 1/2 as fast (30 rpms) or 3 times as fast (180 rpms) the images will be catching the valve stem on the rim at exactly the same position relative to the bottom edge of the side panel of the car and you will convince your brain that the wheel has not moved between each image! As far as "sampling" is concerned-- your brain is imaging (sampling) each frame and recording the changes between the various elements. True-- the wheel is translating along with the car but the rotational position of the wheel is being repeated (sampled) at exactly the same relative position to the body of the car at exactly the same point in each repeating sequence of images! The "sampling" reinforces the mental images which then advise you there is no rotational movement of the wheel (or only very slow foreward or rearward movement) taking place!

Answer 3

think of that the sky's the interior floor of a sphere that surrounds and encloses the earth. Now image status on earth finding up on the sky and picture that the earth is rotating like a precise. because it rotates apparently to you that the sky floor is unquestionably rotating interior the alternative direction so thaaat the celebrities are shifting from east to west.

Answer 4

I read it in a book. Not only in movies does it happen, but even in real life. When a wheel is spinning very fast, it is not easy for our brain to register it so quickly. But it is easier for it to think that it is spinning slowly but backward.