The laser principle we have discussed thus far applies to what is called a three-level laser. The disadvantage of such lasers is that the material they use limits the wavelength of light they can produce. There are lasers such as the free electron laser, one example of lasers that can produce laser beams in a specific wavelength.

Free electron lasers make use of the phenomenon in which electrons traveling straight near the speed of light will release light if their course takes a sudden turn. If you place magnets with their north and south poles facing each other, and then fire in an electron at high speed as shown in the diagram below, the electron emits light while winding at the mercy of the magnetic field. Amplifying the movement of electrons between the mirrors produces a laser beam. A laser beam's wavelength can be varied by changing the energy of the electrons fired in, thus enabling every kind of laser beam, from microwave to ultraviolet lasers.