The Photoelectric Effect
The effect describes the emission of electrons from a surface, typically metal, in reaction to incident light. What happens is that the incident light is absorbed by the electrons (called photoelectric because their excess energy derives from light) within the metal, giving them the needed energy to be released.
Light has wave-particle duality, a prevalent concept in the quantum world that explicates particles possess both wave-like and particle-like properties. Einstein proposed that light consists of photons that interacted with electrons not like waves but like particles. For any frequency of the incident radiation, E=hf (Planck’s constant*frequency) measures a photon’s energy. So, the higher the intensity of the incident light, the more incident photons and the more emitted electrons, each of which would carry the same average energy, presuming that for every absorbed photon is a released electron. To increase energy of the photons and therefore the electrons, we look to the aforementioned equation - f and E are directly proportional, therefore all we’d need to do is increase the frequency of the incident light.
Einstein’s expansion on the photoelectric effect earned him the Nobel Prize and forever changed how physicists viewed light. In other words, this shit’s important.