Lasers are possible because of the way light interacts with electrons.
Electrons exist at specific energy levels or states characteristic of that particular atom or molecule. The energy levels can be imagined as rings or orbits around a nucleus. Electrons in outer rings are at higher energy levels than those in inner rings. Electrons can be bumped up to higher energy levels by the injection of energy-for example, by a flash of light. The wavelength or color of the emitted light is precisely related to the amount of energy released.