This may be niche, but I’ve always wondered. Like if you look at a digital clock while brushing your teeth it’ll look like the numbers are bouncing all around, when nothing else does.
In: Physics
Digital clocks often use [7-segment](https://en.wikipedia.org/wiki/Seven-segment_display) displays with illuminated LED lights making up the segments.
To reduce the total number of digital pins needed on the clock’s microcontroller, the clock’s digits are rapidly illuminated one at a time. Referred to as “multiplexing” the display digits.
The blinking digits are so rapid, our eyes will generally perceive all of the digits as being illuminated without flicker. Buuuuuuut, in dim light, if our eyes ever quickly change where they’re looking, we’ll see persistent traces of the blinking multiplexed digits spread across our field of view.
Automotive engineers botched a related issue in a big way, when LED tail lights first went mainstream back in the past couple decades. If you’re ever behind those vehicles at night, your whole field of view is filled with persistent images of the rapidly blinking LED tail lights.
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