In 1917 Einstein, in the process of providing an especially simple and direct proof of Planck’s radiation equation introduce into physics a new  concept- that of stimulated emission. As it happens this process is basic to laser operation. Even though the first operating laser did not appear until 1960, the groundwork for its invention was put in place by Einstein’s work. The importance of stimulated emission is indicated by the fact that the very word “laser” is an acronym for “light Amplification by the Stimulated Emission of Radiation.” 
It is interesting that, in 1905, Einstein first advanced the concept of the photon and realized that it could provide an explanation for the photoelectric effect. We learn from both of these examples that practical devices of major importance can flow from the study of problems that seem to have no relevance to technology. When next you see a photoelectric elevator door opener or listen to a CD player, think of Einstein.
Now let us examine three processes that involve the interaction between matter and radiation.