The theory provides the bulk of physical explanation and related laws, which govern absorption, emission and spontaneous emission from the ordinary matter. Early laws about thermal radiation and the blackbody emission, such as Rayleigh-Jeans, Wien, Planck laws are cast in a single theory and formalism through the concept of quantized energy at the level of atoms emission/absorption of light. Explain the modern concept of quantum optics and their link to the design of modern devices for the measurements and/or production of coherent light.