Lines that run down (or up) in computers and tv sets?

Question

when we see them on tv (or movie) but do not seem to notice them when we're actually in front of the monitor or screen. Why do the cameras 'see' them and we cannot?

Answer 1

The picture on your TV or computer monitor is made by scanning the screen. The pixels light up, go out, and light up again,  a line at a time. It happens too fast for your eye to see as anything more than maybe a slight flickering, but TV and movie cameras take 30 or 24 still pictures per second and catch one or more lines of pixels while they're off.

Answer 2

TV's and Monitors refreshes its display every n number of milliseconds, we cannot see this because our brains cannot process the signal from our eyes that fast.

Basically, its an optical illusion.