wavelength
Lasers Most light emitted by atoms is polychromaticcontaining more than one wavelength. In contrast, lasers from light amplification by stimulated emission of radiation emitmonochromatic light a single wavelength only. Lasers have many applications in fiber-optic telecommunications, the reading and recording of compact discs CDs and digital video discs DVDs , steel cutting, and supermarket checkout scanners. Laser beams are generated by the same general phenomenon that gives rise to emission spectra, with one difference: only a single excited state is produced, which in principle results in only a single frequency of emitted light. In practice, however, inexpensive commercial lasers actually emit light with a very narrow range of wavelengths. How a CD player uses a laser to read a CD. Inside a CD is a flat, light-reflecting layer called land. On the land are many pits recorded in a spiral-shaped track. From the lasers point of view, pits are actually the bumps shown here because the master disc with pits is duplicated negatively, turning the pits into bumps. Pits have the same light-reflecting surface as land, but there are differences in the frequencies of the reflected light in the pit and the land, making light reflected by pits relatively dark compared with light reflected by land.