Need Help With a Physics Phenomenon (Quick Question)

[Nitrogen]

What's the optical phenomenon called when a wheel is rotating clockwise so fast that it eventually looks like it's rotating counter-clockwise?

Not a homework question, just curious.

 

[Bandersnatch]

Hi Nitrogen. Welcome to PF.

It's an optical illusion usually observed under stroboscopic conditions. It's called the "wagon-wheel effect".
The wiki has got a good article on it:
http://en.wikipedia.org/wiki/Wagon-wheel_effect