Eli5: Why do wheels look like they’re spinning the wrong way when going fast?
In: Physics
It only looks like that in videos and its because the cammera takes like 24 pictures a second, and if the rotor or whatever is spinning in a frequency thazs close to that 1/24th of a second or a multiple of that, you will have each frame of the video show a still image of the wheel or rotor but shifted a bit in one direction. Show these frames at some speed again, aka play the video and you will see the illusion of movement.
You can get a similar effect with stroboscope lights without a cammera.
Ok everyone is answering on video…. but what about in person? Is it the shape of the rim design?? Or am I just slow ?
They don’t. You’re watching film (analog or digital) of a spinning wheel, not a wheel.
Cameras have frame rates, and just like the blinking bulbs on a movie theater marquee appear to move around the rim, what you’re seeing is a frame rate illusion, not movement.
Wheels look like they’re spinning backward when going fast because of a strobe effect from lights or cameras. This happens when the rotation speed matches the frame rate, creating an optical illusion.
The technical term is called aliasing.
https://en.m.wikipedia.org/wiki/Aliasing
It happens because your brain samples (think of it as frames in a video) slower than the circular speed of the rim, so the frequency looks different that actual. In fact you will see a point where it appears the wheel actually stops spinning.
It’s the strobe effect. It rotates 350deg and the camera takes a picture, then it rotates another 350 degrees and the camera takes another picture. Add it up into a video and it looks like it’s going slowly, and basically a 50/50 shot for which direction it’s going.
There are a lot of incorrect answers here.
It’s true that this is observed through cameras but it’s also something that we commonly perceive in real-time too, like when we’re looking at the blades of a helicopter in flight or the wheels of cars out the window on a highway. The formal name for this phenomenon is the [wagon-wheel effect](https://en.m.wikipedia.org/wiki/Wagon-wheel_effect).
The reason why this happens is twofold: It is based both on how the wheels spin and how our brain interprets images. Picture this: A wheel makes one full rotation in 50 milliseconds. But your brain “updates” and interprets your visual cues every 40 milliseconds. So when 50 milliseconds pass, the wheel will spin once fully, but because of the desync in timing you’ll see the wheel *almost* spin once fully. You’ll have to wait another 40 milliseconds because of the timing delay by your brain to see the wheel in its starting position.
Now imagine the same wheel spins twice, which would take 100 ms. At the end of the first wheel spin (as discussed above) you’ll see the wheel slightly behind, and 40ms later because the wheel is still spinning (because 100ms hasn’t elapsed yet) your brain will update itself again, and you’ll see the wheel even further back than you saw the first time.
At this point your brain is going to draw a conclusion. It’s going to interpret all those delays as “Hm, I only see the wheel slightly behind where it originally spins each time. Logically this must mean that the wheel is spinning backwards (opposite), not forward” and it relays this information to you accordingly, causing you to think what you think. If the wheel spins many times quickly (as wheels usually do) you’ll see this effect much faster, creating the phenomenon that you see.
Now it should be noted that in real life, our brain doesn’t have a “fixed” refresh rate. We aren’t computers after all, we can’t easily put a label on our brains. The 40ms refresh rate used above was only for simplicity’s sake, and in reality it constantly fluctuates based on many biochemical processes.
The phenomenon you’re experiencing is called the wagon-wheel effect. It occurs due to the way our eyes and brain perceive fast-moving objects. When a wheel spins rapidly, the brain struggles to process each individual position of the wheel, causing it to blend together. This blending creates an illusion of the wheel spinning in the opposite direction or even appearing stationary. It’s a fascinating visual quirk, and understanding it sheds light on the complexities of human perception.
According to Sniglets, the wheels reach Point Blimfark
POINT BLIMFARK (poynt blim’ fark): n. point at which after spinning fast enough make wheels appear to turn in the opposite direction.
It’s called the “wagon-wheel effect.” Essentially, when the frame rate of a camera doesn’t match up with the rotation speed of the wheel, it creates this optical illusion. Your eyes and brain work kind of like a camera, so you see the same effect in real life sometimes. It’s freaky but fascinating!
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