Bohr model
Bohr's model is also useful because it explains the cause of light. Bohr agreed with classical theory that light has a wave-particle duality (meaning that it is made of both electromagnetic waves and particles called photons), but he said that light was given off when an electron in a hydrogen atom jumps from a higher energy level to a lower one. Basically, if an electron with x amount of energy goes to a lower state of energy and loses y energy, then a photon with energy y is emitted by the atom and either becomes light or some other form of radiation.