Like all light sources, lasers produce light through the action of electrons. When an electron absorbs energy, say, from the addition of heat, it gets excited and moves to a higher-energy orbit around an atom’s nucleus. This excited state doesn’t last forever. When the electron returns to its lower-energy orbit, or ground state, it releases a particle of light called a photon, which also exhibits wavelike properties. The photon's wavelength is determined by the energy difference between the excited state and the ground state. The higher the energy difference, the shorter the wavelength of the photon.

As the energy difference between the excited state of an electron and its ground state increases, so does the energy of the light that is absorbed or emitted. Laser light differs from natural light because the higher-energy light waves that are released share the same wavelength (monochromatic) and are highly organized (coherent). This means that unlike a flashlight, which releases light in many directions, a laser light produces a very strong and concentrated beam (collimated).

Over the years, scientists and engineers have learned to manipulate these characteristics and have developed lasers with greater sophistication. Lasers can operate in visible, ultraviolet, or infrared ranges and be used for various applications. The more photons there are in the laser beam, the brighter and more powerful it will be. Today, lasers are used in digital media devices such as DVD players, in precision measuring and mapping tools, and in machines that cut everything from fabric to steel. They are also ideally suited for use in medical procedures. Because a laser beam can focus so accurately, it can be used for delicate surgical work, such as cutting through and removing tissue. They cauterize, or sear, blood vessels as they cut, generating less bleeding than traditional scalpels and requiring less healing time.

One popular medical procedure that uses laser technology is refractive eye surgery, a technique that reshapes the cornea to change the focal point of the eye and improve vision. By removing just a small part of the cornea, an eye surgeon can flatten out the excess curvature that causes nearsightedness. To do this, the surgeon projects a beam of ultraviolet light onto the surface of the eye. As the beam contacts the corneal surface, it penetrates just a nanometer deep (a billionth of a meter) and vaporizes a microscopic amount.

While lasers can be used to improve health, they can also be harmful to it. The eyes and skin are the most vulnerable body parts. When shone in the eyes, the highly focused energy in a laser beam can produce flash blindness and afterimages—much like what happens if you look directly at the Sun. On the skin, burns can result from the heat some higher-powered laser beams produce. People who work with or around lasers should take basic safety precautions. They should wear protective eyewear, use no more power than is needed to complete a project, keep the beam path from eye level, and keep body parts out of the beam.

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