517. How are some light emitting diodes able to emit more than one color? Can light emitting diodes emit different amounts of light or can they only be on or off?
Light emitting diodes (LEDs) that emit more than one color are actually two different LEDs connected to a single circuit in opposite directions. When current flows in one direction around that circuit, one of the LEDs emits light. When the current reverses directions, the other LED emits light. And when the current reverses directions rapidly, both LEDs emit light alternately. If one LED emits red light and the other green light, then the overall device will appear yellow or orange when they are both operating alternately in rapid sequence. The amount of light that an LED emits depends on the current flowing through it—the more electrons that are falling into holes in the p-type semiconductor, the more light that's being emitted. However, many devices that use LEDs just turn them on or off because that's easier than controlling the current flowing through them. Some day, flat panel displays may use three colors of LEDs—red, green, and blue—in order to present full color images like those on a current television screen. For that scheme to work, the LEDs must be able to emit different brightnesses, so the current flowing through each one must be adjustable.