Explore the fascinating world of quantum radiation in this comprehensive lecture on spontaneous and stimulated emission by Professor G Srinivasan. Delve into Einstein's groundbreaking 1917 paper and his profound insights that led to the development of quantum theory. Examine key concepts such as Einstein coefficients, absorption and emission processes, and inverted population. Investigate two-state systems in quantum mechanics, including Zeeman and hyperfine splitting of energy levels. Discover the intricacies of the hydrogen atom's hyperfine structure and the ammonia molecule's properties. Learn about masers, including ammonia and hydrogen variants, and their significance in astrophysics. Gain insights into celestial masers and their role in understanding the universe. This lecture, part of the "Summer course 2018 - A Random walk in astro-physics" series, provides a deep dive into the physics underlying various astronomical phenomena, from interstellar hydrogen clouds to active galactic nuclei. Read more