Why does a wheel rotating appears to rotate in opposite direction?

The stroboscopic effect can cause the image of a wheel (let us say with many spokes) to appear to be turning at a different speed and even backwards.

If succeeding images of a wheel with 12 spokes are taken when the wheel has made , say one eleventh of a turn, just less than enough of a portion of a revolution to bring one spoke where another spoke had been previously, the wheel will appear to rotate slowly backwards.