system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come when a camera is pointed at a monitor, it looks like there are lines in the picture?
gpt
Your monitor has what's called a refresh rate, measured in hertz (which is just number of times something happens per second).  60 Hz is a fairly common setting.  That means it redraws what's on the screen 60 times every second, but when it does it doesn't refresh the entire screen at the same time.  Instead it will rotate through the rows continually.  So at any given time a few of the rows are being actively redrawn, while others are up to 1/60th of a second behind.  This all happens too fast for the human eye to detect, but when you take a picture it freezes the screen image at the state it was at that exact moment, so you can see where the redraw (the bright part of the picture) is occurring and the darker parts are the area that hasn't been redrawn as recently.