Science Questions

Why does your hand look funny if you wave it quickly in front of the television or a computer screen ? Alex, Jesus Lane, Cambridge

This is due to an effect called strobing. Although the computer screen looks like it is displaying a continuous picture, the image is actually being replaced with a fresh one between 65 and 100 times a second, so the screen behaves a bit like a large camera flash, taking 100 pictures per second. Since our eyes can only see things that are flashing at less than about 25 times per second, we see a continuous image on the screen. So what happens to your hand ? When you wave the screen is essentially taking a series of rapid snapshots of your hands, giving the bizarre impression that your fingers are jumping across the screen, and that you can see through them.

June 2002