Let's look at the properties of the atom and how the atom interacts with light. I’ll use this knowledge to simulate a working laser. We will learn how LASERs relies on Stimulated absorption, Spontaneous emission, and most importantly Stimulated Emission- This last type interacts with an excited atom, causing it ti relax/deexcite. It will send out a photon with the same phase, frequency, polarization and direction. We exploit this principle in a optical cavity, with the atom as a gain medium. It turns out the pump will cause stimulated emission for the wrong excited state and we need to use a 3 level atom in order to obtain population inversion. I'll also cover meta-stable state, phosphorescence, radiationless transitions and photoluminescence.