What Does LASER Stand For?

LASER stands for Light Amplification by Stimulated Emission of Radiations. Consider an atomic system with three energy states E1, E2, and E3. E1 is a ground state, E2 is a metastable state and E3 is an exited state. Initially, the atoms are pumped from the ground state to excited state E3. The electrons will jump down to the E2 causing spontaneous emission. After some time number of electrons in metastable state becomes greater than ground state, this is called population inversion. Now the triggering photons on this system will produce stimulated emission. In this way a three level laser work.