Science Buddies: "Ask an Expert"

Why is it that sometimes when viewing a revolving wheel it appears to slow, then stop, then begin to revolve backwards? Until recently I had only observed this on film or television and had assumed it was something to do with the refresh rate of the TV screen or the frames per second of the cine film. However I have recently noticed it in the real world. I have an off-road bicycle with very knobbly tyres and I have found that by increasing my speed I can make the knobbles on the front tyre slow down, then stop and then, with a little more effort as I speed up, appear to revolve in the reverse direction. Is the 'refresh rate' theory correct and does my vision come with something similar as it is processed by my brain or is there an entirely different explanation?

Sorry if I've posted this to the wrong group. I'm not a high school student but a 67 year old pensioner still looking for answers.

hippychris

Hello,

The optical phenomenon that you're talking about is something called the wagon-wheel effect. If you look it up on Wikipedia or Google, it'll provide you a thorough explanation of what's happening. Basically, the reason why the wheel looks like it's slowing down or going in the opposite direction is because of something called beta movement where 2 similar objects in different positions, but are in the same line of sight, appear to be in motion in a certain way. Your eyes are tricked by this and so this can be dangerous as some machinery can look like it's going slowly or even off when it's not in reality. Hope this explanation helped!