Stroboscope

A stroboscope is a device that makes moving objects appear to slow down or stop. Using the flashing light version you can watch an arrow drawn on a card and spun by an electric motor.

If the stroboscope flashes at the same speed that the disc rotates you see one arrow .
If the stroboscope flashes twice as fast you see two arrows as the disc has only rotated half a turn between flashes. If it flashes three times as fast then you see three arrows and four arrows if it is rotating four times as fast.

The animation shows the case where the flashing light does not quite match the speed of rotation of the disc and so the arrow rotates slowly.