How is light emitted?
You undoubtedly have seen the light emitted by hot objects. Whether it is the dull red glow of the heating coil on an electric range or the orange glow of the element in an electric oven or the bright yellow-white of the glowing filament of an incandescent lamp, you have seen light emitted by hot objects. Even the yellow glow of a fire comes from light emitted by hot carbon particles. Energy, usually from stored chemical energy, is converted into thermal energy. That energy is transferred to the surroundings by radiation, including light. Unfortunately, producing light in this way is very inefficient because about 97% of the energy goes into infrared radiation that warms the environment rather than light that can be seen. Because of their large energy use, many countries will be banning incandescent lamps in the next few years.
Light can also be emitted by gases and solids. Neon signs are one example of a gas that glows because electrical energy is converted into light energy. High-intensity lamps use either sodium or mercury vapor to produce intense light. Fluorescent tubes and compact fluorescent lamps (CFLs) use electrical energy to excite mercury atoms. The ultraviolet emitted by these atoms causes compounds deposited on the inside surfaces of the lamps to glow. The colors can be chosen to emulate incandescent lamps or daylight. CFLs convert up to 15% of the electrical energy into light.
Lasers, mostly used today in CD and DVD players and supermarket bar-code scanners and pointers, usually consist of a small crystal composed of a mixture of elements like gallium, arsenic, and aluminum. The lasers produce single-color, intense light that is emitted as a compact ray. The LED lights that are often used as on/off indicators, traffic lights, car taillights, stop, and turning lights also use electrical energy and the same kind of materials used in laser pointers to produce light that is radiated into many directions. White LEDs that are beginning to be used in home lighting are costly to produce, but are much more efficient and last much longer than incandescent lamps.