How laser work??

Laser is a device which produces a highly concentrated monochromatic, coherent and unidirectional beam of light. The way it does is easy if we understand few simple things:

Induced absorption:

We know that the electrons takes discrete energy level to change their orbits. The source of that energy can be alot of things. Photon is one of them. The energy carried by a photon is hf . If a suitable photon of energy hf strikes the electron it absorb it and moves to higher orbit. This absorption is called induced absorption. 

Einstein postulated that the rate of this absorption is directly proportion to the number of atoms present in ground state and the density of incident radiation.

i.e.

 dNe/dt=Bi Pr Ng               where Ne is the n. of atoms in excited state 

                                                 and Ng is the n. of atoms in ground state

                                                 and Pr is the density of radiations 

                                                 and Bi is just a constant named einstein coefficient of induced                                                                 absorption

  which is very logical if you look give some thoughts.

If density of radiation then more electron are knocked off to higher orbits and if the n of atoms in ground state is high more electron goes to higher orbit.

Then next thing is these excited tend to return to the stable state which is now known as 

Spontaneous emission:

The excited atom emit a photon while coming back to the stable state spontaneously in the absence of external electromag.netic radiation. These emission are called spontaneous emission. The cause of this emission may be the due to the interaction of moving electron with the magnetic field produced by other atoms of even electron. I remember reading it is caused by the interaction between atoms and vacuum fluctuations of the quantized electromagnetic field. I don't know what it means of may be it refers to the those small electromagnetic forces involving moving electrons .Not sure though. I would love to discuss this.

 So the energy of the emitted radiation is the difference between two energy states.

 Again according to Einstein

dNg/dt= A Ne                        where A is the einstein's coefficient for spontaneous emission

Now we just need to learn about one more type of emission 

Induced or Stimulated emission:

Emission of photon by an atom in the presence of electromagnetic radiation is called Induced or Stimulated emission. When an atom absorb the photon it become excited and then emit photon. The rate of emission is equal to the rate of absorption in equilibrium since both these rate are dependent on the density of the electromagnetic radiations. 

When an atom in an excited energy state E₂  interacts with the radiation of right frequency f , it emits a photon of the same frequency f. In this case atoms are excited using electric discharge. So that the the electromagnetic radiations are not absorbed any further.

According to Einstein, the rate of stimulated emission is directly proportional to Ne and energy density of the radiation . Then the formula becomes

dNg/dt=B₂₁ P Ng               Where B₂₁ is the einstein coefficient of stimulate or induced                                                                        emission. It is similar to B in induced absorption.

The coefficient B₂₁ is used here to address the emission of radiation by the atoms present due to induced emission other than from electromagnetic radiations.

Ok we can all combine our knowledge so far to create a laser:

We just need to stimulate atoms so that the majority of the atoms are in excited state. This state is also called population inversion. Then we have to find a way to make the rays parallel coming out of the laser.

Some early examples of laser are ruby laser and helium-neon laser .

Hope you like it. Follow if you want to read how the ruby and helium-neon laser works.

Give me suggestions in the comment. And let me know if there is anything you want to discuss.