What is the stroboscopic effect?

The stroboscopic effect is a phenomenon due to interrupted illumination of a moving object. When an oscillating body is seen in periodically interrupted light (a series of light flashes occurring at a definite rate) it appears different. If the periodicity of the vibrating body is same as that of the flashing light, the body appears to be stationary. In movies, when a wagon with spiked wheels comes to a stop, the wheels often appear to stand still, then turn backward, stop, turn forward, and then stop again. This is due to the stroboscopic effect. A stroboscope is a light source emitting light with different but definite interruption rates. It is used to determine the frequency of a vibrating body or revolutions per minute of a rotating body. Car engines are tuned with the help of stroboscopic lights.