system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come when you film a computer or TV screen it appears to have black bars running through it that you never normally see?
gpt
Simplest explanation is because of the way old monitors/TVs display their image at a slow frame rate and how the camera is recording at a different frame rate. During playback they’re out of sync so you start to see partial scans on the display. 

Kinda like how a fast spinning wheel can look like it’s spinning backwards.