From Wikipedia

stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. The principle is used for the study of rotatingreciprocating,oscillating or vibrating objects. Machine parts and vibrating strings are common examples.

Here, I wrote a small PIC programme to flash a Led light at a specified frequency. The frequency can be changed by setting a variable resistor. The duty-cycle is set to near 1% so as to minimize the blurring. When run, a rotating CPU colling fan is seen stationary, when its rotation frequency and the Led blinking frequency is exactly matched. Since, the light intensity of the led light is pretty low, the room needs to be dark for the effect to work.

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