Stroboscope principles

How does a stroboscope make a moving object look like it’s standing still?

A stroboscope is an instrument that emits a series of brief, intense flashing lights at specific intervals.

When the flashing light from a stroboscope is directed onto an object rotating at high speed (e.g., a cooling fan inside a PC), the moving fan appears to stand still.

The stroboscope makes the fan rotating at high speed appear to stand still due to the visual persistence*1 of the human eye.

When the frequency of the flashing lights from the stroboscope is adjusted to synchronize with the fan rotating speed, the number of apparently stationary fan blades you see corresponds to the actual number of fan blades.*2

*1 The retina of the human eye retains an image for approximately 1/16th of a second. The stroboscope relies on this characteristic of the human eye to provide its intended function. Projectors used in movie theaters also rely on characteristics of human vision to display motion pictures.

*2 If the stroboscope emits a series of flashing lights at a frequency faster than the fan rotating speed, you will see more fan blades than there actually are. In contrast, if the frequency of the flashing lights is slower than the fan rotating speed by a fraction of a whole number, you will see the actual number of fan blades. For more detailed information, see "Using the Stroboscope" pages.