A stroboscope is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It consists of light source which produces brief repetitive flashes of light. If a body rotating at some frequency is illuminated with a light flashing at the same frequency, it is visible only when it is at a particular position. This gives an impression that it is stationary. If the two frequencies differ slightly, the body will apprear to move very slowly.
Observation of a rotating body with a periodic flashed light, using a disk with a marker.
When the frequency of the phenomenon under observation and the frequency of the flashing light are matching, one can see a still image.
What happens when the frequency of the light is slightly increased, or slightly decreased?
What happens when the frequency of the flashing light is twice the frequency of the phenomenon? and when it is the half of its value?