carrier signal A signal of a single frequency that can be altered by a second signal that contains data. The high-pitched tone that a modem emits is an example of a carrier signal. Changes, or modulation, in the carrier's amplitude, frequency, or phase can convey information. crystal A structure with a geometric form that reflects the geometric ordering of the atoms or molecules comprising it. Examples are salt, sugar, and snowflakes. fiber optics The branch of optical technology that deals with the transmission of light or images across short or long distances, through transparent fibers. frequency The number of times a repeated event occurs during a given time period. Frequency is often measured in units of cycles per second. One cycle per second is called a hertz (Hz). laser An acronym for Light Amplification by Stimulated Emission of Radiation. The light waves in a laser move in exactly the same way as each other and are all the same frequency. This results in a powerful beam of light that can produce intense heat when focused at close range. Lasers are used in many fields. In medicine, lasers are used in microsurgery, cauterization, and for diagnostic purposes. Lasers are used in telecommunications to transmit data. lattice The regular pattern of points in a crystal that atoms, molecules, or ions vibrate around in a crystal. maser An acronym for Microwave Amplification by Stimulated Emission of Radiation. A device that can produce coherent electromagnetic radiation in the microwave frequency band; used in atomic clocks and to amplify weak radio signals. Masers which operate at optical frequencies are called lasers. optics The science that deals with the properties and phenomena of both visible and invisible light and with vision. oscillator A device that generates a periodic variation of electric current of a particular frequency in an electronic circuit. photons The quantum of electromagnetic energy, a discrete particle having zero mass and no electric charge. quantum Discrete unit of electromagnetic energy such as that produced by an electronic transition in an atom. semiconductor A material, usually crystalline, with electrical conductivity that is intermediate between that of metals and that of insulators. The conductivity varies depending on the amount and chemical nature of impurity elements incorporated during the manufacturing process. The ability to control this variation is the basis for the operation of all semiconductor devices such as transistors, light emitting diodes, and injection lasers. spectroscopy The study of the characteristic frequencies of atoms or molecules by observing the radiation they emit or absorb. spectrum The distribution of some physical property, arranged in numerical order. One example is white light split by a prism. The resulting spectrum is arranged in order of increasing wavelength from violet, with the shortest wavelength, to red, with the longest. thallium A soft, malleable, rare, bluish-white metallic element; used in the manufacture of alloys and, in the form of its salts, in rodent and ant poison. transistor A semiconductor device that amplifies, oscillates, or switches the flow of current between two terminals by varying the current or voltage between one of the terminals and a third.