The coherent light produced by a laser differs from ordinary light in that it is made up of waves all of the same wavelength and all in phase (i.e., in step with each other); ordinary light contains many different wavelengths and phase relations. Both the laser and the maser find theoretical basis for their operation in the quantum theory. Electromagnetic radiation (e.g., light or microwaves) is emitted or absorbed by the atoms or molecules of a substance only at certain characteristic frequencies. According to the quantum theory, the electromagnetic energy is transmitted in discrete amounts (i.e., in units or packets) called quanta. A quantum of electromagnetic energy is called a photon. The energy carried by each photon is proportional to its frequency.

An atom or molecule of a substance usually does not emit energy; it is then said to be in a low-energy or ground state. When an atom or molecule in the ground state absorbs a photon, it is raised to a higher energy state, and is said to be excited. The substance spontaneously returns to a lower energy state by emitting a photon with a frequency proportional to the energy difference between the excited state and the lower state. In the simplest case, the substance will return directly to the ground state, emitting a single photon with the same frequency as the absorbed photon.

In a laser or maser, the atoms or molecules are excited so that more of them are at higher energy levels than are at lower energy levels, a condition known as an inverted population. The process of adding energy to produce an inverted population is called pumping. Once the atoms or molecules are in this excited state, they readily emit radiation. If a photon whose frequency corresponds to the energy difference between the excited state and the ground state strikes an excited atom, the atom is stimulated to emit a second photon of the same frequency, in phase with and in the same direction as the bombarding photon. The bombarding photon and the emitted photon may then each strike other excited atoms, stimulating further emissions of photons, all of the same frequency and all in phase. This produces a sudden burst of coherent radiation as all the atoms discharge in a rapid chain reaction. Often the laser is constructed so that the emitted light is reflected between opposite ends of a resonant cavity; an intense, highly focused light beam passes out through one end, which is only partially reflecting. If the atoms are pumped back to an excited state as soon as they are discharged, a steady beam of coherent light is produced.

The physical size of a laser depends on the materials used for light emission, on its power output, and on whether the light is emitted in pulses or as a steady beam. Lasers have been developed that are not much larger than a common flashlight. Various materials have been used as the active media in lasers. The first laser, built in 1960, used a ruby rod with polished ends; the chromium atoms embedded in the ruby's aluminum oxide crystal lattice were pumped to an excited state by a flash tube that, wrapped around the rod, saturated the rod with light of a frequency higher than that of the laser frequency (this method is called optical pumping). This first ruby laser produced intense pulses of red light. In many other optically pumped lasers, the basic element is a transparent, nonconducting crystal such as yttrium aluminum garnet (YAG). Another type of crystal laser uses a semiconductor diode as the element; pumping is done by passing a current through the crystal.

In some lasers, a gas or liquid is used as the emitting medium. In one kind of gas laser the inverted population is achieved through collisional pumping, the gas molecules gaining energy from collisions with other molecules or with electrons released through current discharge. Some gas lasers make use of molecular dissociation to create the inverted population. In a free-electron laser a beam of electrons is wiggled by a magnetic field; the oscillatory behavior of the electrons induces them to emit laser radiation. Another device under development is the X-ray laser, which presents special difficulties; most materials, for instance, are poor reflectors of X rays.

The light beam produced by most lasers is pencil-sized, and maintains its size and direction over very large distances; this sharply focused beam of coherent light is suitable for a wide variety of applications. Lasers have been used in industry for cutting and boring metals and other materials, and for inspecting optical equipment. In medicine, they have been used in surgical operations. Lasers have been used in several kinds of scientific research. The field of holography is based on the fact that actual wave-front patterns, captured in a photographic image of an object illuminated with laser light, can be reconstructed to produce a three-dimensional image of the object.

Lasers have opened a new field of scientific research, nonlinear optics, which is concerned with the study of such phenomena as the frequency doubling of coherent light by certain crystals. One important result of laser research is the development of lasers that can be tuned to emit light over a range of frequencies, instead of producing light of only a single frequency. Work is being done to develop lasers for communication; in a manner similar to radio transmission, the transmitted light beam is modulated with a signal and is received and demodulated some distance away. Lasers have also been used in plasma physics and chemistry.