Beginings...

1900. Marck Planck stated his hipotesis that ionised atom emmits energy in energy pack which he called quants, and not all time like electromagnetic radiation. Planck never completed his theory and problems that conserned it. So 5 years later Albert Einstein stated his theory telling that light is in fact made from photons and not from waves like scientist thought. He told that electron at special enviroment can apsorb and emmit photons. For this theory he got the Nobel prize for explaining 'Fotoelectric efect'. Not everbody was pleased with Einstein's theory about light like particle and disscusions lasted for years. For this time Einstein discoverd a new fenomenom . With the model of atom by Niels Bohr in 1913., electron that is moving in atom has a path wich depends on energy of electron. Electron can apsorbe energy that he needs for moving to place with higher energy, and when it is going back to state of lower energy he emmits energy in form of photons. That atoms in higher state, wich electrons have more energy, with time will go back in elementary state, and while doing that they will release extra energy in form of photons. In this model random emission starts so direction of spreading photos is random. Einstein realised that if photon would hit the atom that is ionsed, atom would release photon so it could use incomming photon. Therefore, released atom has same path like recieving photon, so that conditions can start a chain reaction. But here is one trick, in this space must be more ionised atom then atoms whit lower energy. So before transmiting photon is this space atoms are ionised with light.