“A simple tool to measure speeds of rotating objects
Stroboscope is a flashing light that can provide light pulses at a variable rate. When a rotating object is observed in a powerful beam of pulsed light with the frequency of that pulsed light matching the rotations per second of the rotating object, the rotating object appears stationary.
This allows the speed of any rotating object to be calculated by varying the pulse rate until the rotating object appears stationary. Once this state is achieved, revolutions per minute (rpm) of the rotating object can be calculated from the frequency of the pulsed light.
If the object appears to be stationary when the pulsed light is at a frequency of 50Hz or 50 times a second, the object is rotating at 3,000 rpm (50 x 60).”