The human eye retains an image for a short while after the actual image has been removed.

If the oscillations of a light turning on and off, are faster than which the human eye can detect change, the light appears to be permanently turned on.

The duty-cycle refers to the period of time that the light is turned on. If a light is on for half of the time, the duty cycle would be 50%. In this case, if the oscillations are fast enough, the light will appear to be on for the full duration of the on off cycle, but at half of the actual brightness of when it’s turned on.