866. When TV screens or computer monitors are shown on television shows, they flicker or bars of light wave across them. Why does this happen? — SY, Halifax, Nova Scotia
Although you can't tell it by looking at a television screen, the image on that screen is formed one dot at a time by beams of electrons that are scanning back and forth across its surface from inside. The image is built one line at a time, from the top of the screen to the bottom of the screen, and each line is itself built one dot at a time, from the left side of the screen to the right side of the screen. You can't see this sequential construction process because your persistence of vision prevents you from seeing any changes in intensity that occur in less than about 1/100 of a second. In any short period of time, the screen will only have had time to produce a few horizontal lines of dots. When a camera or television camera observes a television screen, it often makes its observation in such a short period of time that only part of the screen is built. When you then look at the recorded image, you see a horizontal bar of image—the portion of the image that was built during the observation.