In this chapter, three problems whose resolution laid the foundation of quantum mechanics are discussed. First the pioneering work of Max Planck is described who explained the spectrum of light emitted from a so-called blackbody by making a bold ansatz that the energy associated with the oscillations of electrons comes in packets or quanta of energy. Second it is shown how Einstein invoked Planck’s hypothesis to explain the photoelectric effect by arguing that light should come in packets or quanta of energy and this energy should be proportional to the frequency. Electrons are emitted if the energy of the quantum of light, which has come to be known as a photon, is higher than a critical value. The third problem relates to the atomic structure of hydrogen. A full description is given of how Bohr applied a quantization condition to explain the emission of light at certain specific frequencies.
This concluding chapter explains that the modern theory of light falls within the field of quantum mechanics. At first glance, quantum mechanics does not seem that strange—its name is based on the fact that light comes in units and that electrons have discrete energy states. It also includes the uncertainty principle, which states that one cannot know certain pairs of physical properties with perfect precision. Moreover, quantum mechanics involves the wave-particle duality of photons. The chapter then explores two of the most unusual aspects of quantum mechanics: two-slit interference and quantum entanglement. Both violate the most fundamental notions about how the world works.