system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do computer screens have that flickery bar going through them in tv/movies?
gpt
Old school CRT monitors display an image by having a electron beam scan across the back of the screen. Pixels light up when the beam hits them and then fade again until the beam comes back around.

This is happening with a certain frequency, for example 60 times per second. It's good enough for our eyes since they don't adjust fast enough to really see the pixels fade out and light up over and over. Although higher refresh rates do feel easier on the eyes.

A camera recording a picture records an image (a frame) x times per second. For example 25 frames per second. So the camera isn't recording non stop, it's only recording x amount of times per second. And the pixels in the monitor aren't brightly lit non stop, they're only bright when the electron beam hits them before dimming again.

This means that every time your camera takes a frame, a row of pixels on the monitor will be brightest (where the beam hits) while pixels further away from the beam are dimming. Your eyes don't see it, your camera does.

Usually the electron beam moves at a different frequency than your camera records frames. So every time the camera records a frame, the electron beam will be lighting up a different row on the monitor. This causes the brightest row to be in a different place every time, when those frames play back fast as video, it looks like a flickering bar is moving back and forth across the monitor.

You can actually stop this effect by synching the camera's frequency with the frequency of the monitor so the brightest part of the monitor is always in the same place when the camera records a frame. You'd have to synch the camera's frequency with that of the monitor though which often doesn't happen when filming on location.

We don't really use old fashioned CRT monitors anymore but most screens still have a refresh rate of some kind.