Pssst! Wanna see a scan line?

1. Stand back from your computer monitor, about 3 metres (10 ft.).
2. Tilt your head horizontally, right or left.
3. Pick two points: one on the ceiling above your monitor, the other on the floor below.
4. Look at the ceiling point, then suddenly snap your eyes down to the floor.

Did you see a bright white line on the monitor? If so, you just saw a scan line.

What is a scan line?

Most monitors paint pictures in light, using just one horizontal line at a time, starting at the top and quickly scanning to the bottom.

That scan line moves down too fast for you to see it, usually. But if you sweep your eyes fast enough across the screen, they can almost keep up.

The result? You see a very bright white line (why is it so bright?) that seems to stand nearly still.

The faster you move your eyes, the slower the scan line seems to move and the narrower it looks.