In 1917 Albert Einstein considered a different possibility. He came to the opinion that there can happen also another physical process called stimulated emission. It is quite different from the first situation. What does it consist in? Well, let's imagine an excited atom. As you know it emits a photon after a moment. The emitted photon meets some other excited atom. But this time the photon makes the atom emit an identical photon and so the atom gets back to the ground state. Yet there are two identical photons: the first one - forcing, and the second one - forced. They are moving in the same direction, have the same phase and frequency. Such process is the stimulated emission.

As you have probably noticed a forcing photon must have identical energy with a photon that would be emitted by the second excited atom when getting back to the ground state. And what happens to the photons yet? Well in their parallel paths they could meet another excited atoms. So after they cause stimulated emissions in two atoms, there are four identical photons moving in the same direction. And so on; the process grows like an avalanche. Well at least in this case; sometimes it fades fast when too many photons get absorbed meeting not excited atoms or atoms at different energy states. But there is some instrument causing avalanche stimulated emissions. It is the laser.

To proceed the avalanche emission needs most of the atoms in the laser to be excited (such situation is called the inverted population). Usually when we deal with a group of atoms, most of them are in their ground state (the normal population). To make the electrons of the atoms in the laser achieve a higher state short, highly energetic scintillations having proper length are used. The electrons absorb the photons and jump on the higher orbit. Then the electrons emit thed photons having some lower energy. And so the electrons jump down but not on the initial orbit (with other words - not to the ground state). The substance used in the laser is a specially chosen one, so the lower orbit of the achieved excitation is the metastatic one, which means that the electrons can stay there for some, quite long period of time. And finally some of the electrons jump down from the metastatic orbits to the ground state. Photons they emit cause next emissions and so the process grows avalanche.

The gas in the laser is closed in an oblong tube. At its both ends there are mirrors. One of them is partially transparent, what makes some of the photons leaving the instrument as a coherent beam. The rest is reflected, and so it gets back causing next stimulated emissions.

Light emitted by lasers is completely different from light emitted by other sources. Laser light is coherent - all its photons have the same energy. That is why laser light is not white, but for example red, green or blue; the light beam consists here of only one wavelength precisely the same at all photons. There are also lasers emitting light that is invisible for human's eye. Such emission can be observed because it stimulates some substances to glow. Laser light as it is coherent has many applications:
   - It is widely used in medicine; for example it is used instead of a traditional scalpel.
   - It is used for reading-out CDs and in other electronic equipment.
   - Lasers are used for precise treating and forming various materials.
   - Thanks to lasers we can precisely measure distances to far-away objects. For example the Apollo crew have left special mirrors on the moon to reflect a laser beam sent from the Earth. That way the distance to our natural satellite has been measured very precisely.
   - Lasers are used in army as target positioners or even as a weapon. A coherent beam of high energy can destroy distant objects like satellites or rockets.
   - Of course, for scientists laser is a very useful instrument for conducting many experiments.
   - And one more example is light pipes, which you probably have much heard of lately, as they are more and more commonly used in telecommunication. And so in light pipes there is laser light used.