system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 Spinning car tires look like they are going backwards, why?

I know it is an optical illusion but I have no idea how it works.
gpt
Let's say you have an analog clock that only has the seconds hand making a full spin every 60 seconds. You put a camera to take images of it at set intervals. Then you review the captured images in sequence.

Let's say you set the camera exposure to every 59 seconds. The first image has the second hand at 59, the next at 58, and then 57, etc. Even if the second hand is spinning clockwise, the camera images played in sequence makes it look like it's spinning backwards!

This is known as aliasing where the sample frequency (inverse of time between captured represented as 1/seconds) is not fast enough to capture the the repeated motion of an object (spinning hand).

To capture the true direction, you have to sample faster. If you capture at 30 second intervals, you will have alternating up and down hands at 0 and 30 seconds. How can you tell the direction if it spins clockwise or counter? Capturing at 29 seconds will allow you to figure it out. Clockwise spinning hands will give 29, 58, 27, etc. Counterclockwise will give 31, 2, 33. Even though the movements are large, you can see the direction of the hand.

You need to sample at less than half the object's frequency to figure out the true direction. This is known as the Nyquist's frequency.