wheels appear to turn backward

Factual Questions
1

I don’t think there have been any threads about this. Sometimes in TV and movies, a spinning wheel appears to be turning backwards. This happens because the wheel rotates forward one spoke (if there are 8 spokes, thats 1/8th of a revolution) in just slightly less time than the time between frames (24 frames/sec for TV and 30 for movies or something like that). So, its as if the wheel moves backward slightly each frame, and then gives the illusion of turning in reverse. However, I saw this same effect happening in real life. I was just looking out the car window and the wheel of another seemed to turn slower and slower then eventually start turning backward, just like it looks like in movies/TV. Do our eyes take a specific “snapshot” at regular intervals? Why did this happen?

2

Nope, brains don’t have a system clock. You need strobe light to create the effect.

It can happen at night on well-lit roads. If the streetlights are not incandescent bulbs but instead are mercury-vapor (white) or sodium vapor (pink), those lights are actually flashing on and off 120 times per second. They act like a strobe light. They briefly go almost dark, so any “backwards wheels” that you see will be fairly fuzzy. Not like a true strobe which can “freeze” a moving object because the light only turns on for less than a thousandth of a second.

3

I can think of two reasons.

  1. Your eyes cannot stay fixed on any point of focus for more than a fraction of a second, even though you are concentrating on a fixed point in space. They dance arounf the main focus. This would give the effect of movement with and against the spinning object that produces the effect.

  2. Blinking

4

Cecil Adams on How come the wheels of a moving car appear to rotate backward sometimes?

5

There was a thread on the subject as well, because I remember posting to it, and what I posted was that I had observed the same thing (seeing wheels appear to spin backwards in real life, e.g., looking out car windows at the wheels of other cars in parallel lanes on the highway). And I’ve seen it in broad daylight.

6

Maybe it has something to do with the fact that you’re simultaneously squinting and producing the “raspberry” sound with your lips. :smiley:

7

I’ve seen it in broad daylight, and other people in the car couldn’t see it. Maybe there are two kinds of people in the world…