Eli5: Why is it when you see a propeller (like from a turbine airplane) starting up it seems to switch directions as it spins faster and faster ?

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Eli5: Why is it when you see a propeller (like from a turbine airplane) starting up it seems to switch directions as it spins faster and faster ?

In: Physics

You typically don’t see this with your eyes, but do with a video camera. It has to do with aliasing, which means you aren’t taking pictures fast enough. If your camera is doing 30 fps, it takes one picture every 1/30th of a second. If the propeller is spinning at 30 rotations per second, it will appear to be still, because it will be in the same spot every time the camera takes a picture. If it’s going a little bit less than 30 rotations per second, it will actually appear to move backwards because it rotated *almost* all the way around by the time the next picture is taken.

This has to do with something called the nyquist criterion, which states that you can only detect frequencies less than half of your sampling frequency.

To see why this is, imagine you’re watching an analog clock on the wall. The second hand goes around once every 60 seconds. Normally, you see the second hand move in small jumps, so you can be confident that it’s moving clockwise because the current position is only a bit ahead of the last one.

If you hold your eyes closed and open them once every 25 seconds, you’ll see the second hand at the 12, the 5, 10, 3, etc. even though the jump between peeks is bigger, you can apply the same logic and deduce that the second hand is moving clockwise.

If you peek once every 30 seconds, you’ll see the second hand alternate between 12 and 6. Even though you know the second hand moves clockwise, there’s nothing in what you’re seeing that could actually confirm one way or the other.

Let’s now lower the sample rate even more, down to once every 35 seconds. You’ll see the second hand go from 12 to 7 to 2 to 9. You’ll notice these are precisely mirrored from the numbers in the 25 second case, or, you could say, it appears to be moving backwards.

The same effect happens when the frequency speeds up to surpass half the sample rate, as when the sample rate slows down to less than half the frequency.

Not exactly what you are asking about, but check out this [Smarter Everyday ](https://youtu.be/dNVtMmLlnoE) video which shows why crazier things happen due to the same reason already explained.