Albert Einstein (1879-1955) built further on this pivotal idea and it led him to the discovery of the photo-electric effect in 1906. In fact this work got Einstein his Nobel Prize. He showed that the reverse of Plānck’s hypothesis of energy quanta is also true. Light shining on certain sensitive metals generates a small electric current in them. This current varies not with light intensity but with changing light frequency. Mathematically, Einstein showed that light is absorbed too in discrete amounts or quantum packets of energy, and these were the same photons of Plānck’s Black-body radiation.
As the work progressed in early 1900s, light was established to have a dual nature. This came to be known as the wave-particle duality and initially was a very confusing concept. Neils Bohr, one of the co-founders of the atomic theory, accepted this duality and extended it to the ‘electron’. He considered the Hydrogen atom and its light-emission spectrum. The discrete lines of different light frequencies in this spectrum pointed towards discrete energy levels for the planetary electron orbiting the hydrogen nucleus. This was like saying that a planet can orbit the sun in a specific orbit only with all other possibilities being forbidden or out of bounds. Bohr further showed that the ‘electron’ is not only particle–like in its behavior but it has wave-like characteristics too under certain conditions. In the case of the Hydrogen atom the electron assumes only those orbits which are integral multiples of its natural ‘wave-length’, λe. That is it accepts only those orbits which have circumferences of ‘n’ times λe where n= 1,2,3 and so on. This theory gave a brilliant first correspondence to the energy levels experimentally observed in the hydrogen spectrum and this hypothesis gave rise to atomic Quantum Theory, rest is history. Schrödinger, Dirac, Pauli, Heisenberg and many other minds contributed in quick succession to cement this theory into one of the major scientific breakthroughs of 20th century.