Equation # 3: Quantum energy of a photon
When trying to explain the cavity radiation(or blackbody radiation), Planck hypothesized that the radiation is absorbed or emitted only in discrete quantities, unlike classically accepted model of continuous waves of light. Einstein extended this idea to all electromagnetic radiation and suggested that all electromagnetic radiation, from X-rays to visible light to microwaves to radio waves, all kinds of “light” exhibits the discrete (or quantum) nature. There is a lot to this story which is very interesting and shows how limited our assumptions are, regarding the basic laws of nature.
For a quirky explanation, see https://www.youtube.com/watch?v=H-7f-3OAXm0
For more equations, see Famous equations in Physics