Television

A cathode ray tube contains a positively charged region (the anode) and a negatively charged region (the cathode). The cathode is located at the back of the tube. As electrons exit the cathode, they are attracted to the anode. The electrons are also focused electronically into a tight beam, which passes into the central area of the television screen. The central region is almost free of air, so that there are few air molecules to deflect the electrons from their path. The electrons travel to the far end of the tube where they encounter a flat screen. The screen is coated with a molecule called phosphor. When an electron hits a phosphor, the phosphor glows. The electron beam can be focused in a coordinated way on different part of the phosphor screen, effectively painting the screen (a raster pattern). This process occurs very quickly—about 30 times each second—producing multiple images each second. The resulting pattern of glowing and dark phosphors is what is interpreted by the brain as a moving image.

Black and white television was the first to be developed, as it utilized the simplest technology. In this technology, the phosphor is white. Color television followed, as the medium became more popular, and demands for a more realistic image increased. In a color television, three electron beams are present. They are called the red, green, and blue beams. Additionally, the phosphor coating is not just white. Rather, the screen is coated with red, green, and blue phosphors that are arranged in stripes. Depending on which electron beam is firing and which color phosphor dots are being hit, a spectrum of colors is produced. As with the black and white television, the brain reassembles the information to produce a recognizable image.