Now, in this lecture, I'm going to give a very, very brief introduction to quantum mechanics. The reason this branch of physics is important in chemistry is it becomes important when we talk about quantum numbers and the photoelectric effect. Now, quantum mechanics is the study of microscopic phenomena such as electromagnetic magnetic waves and the study of subatomic particles such as electrons, protons, neutrons, et cetera. Now, the main idea, the main conclusion that we get from quantum mechanics is the following tiny subatomic particles called electrons and protons and neutrons and other particles. Now, subatomic simply means smaller than an atom gain or lose energy in discrete amount called photons. Now, we're going to talk more about photons and what they are when we'll talk about the photoelectric effect.