Quantum Theory

This chapter deals with the important experimental studies on black-body radiation, Wein’s law, Rayleigh–Jeans law, Planck’s distribution law, etc. The black-body radiation studies reveal that the maximum wavelength of radiation 1 is inversely proportional to temperature T. The expression for Wein’s law has been deduced on the basis of thermodynamic considerations. The law gives a clue to understand that it holds true only at low temperatures and short wavelengths. The Rayleigh–Jeans law is based on the general theorem of elasticity and is in excellent quantitative agreement with experimental observations for large wavelengths and high temperatures, leading to ultraviolet ‘catastrophe’. Actually, Max Planck enunciated the correct form of the temperature–radiation law based on some assumptions and finally proposed Planck’s quantum theory. This chapter also explains the photoelectric effect and important conclusions drawn on the basis of experimental facts. Einstein applied the quantum theory to explain the photoelectric effect and to define threshold frequency. On the basis of plot of frequency of radiation versus voltage applied, a linear graph was obtained, which enabled finding the value of Planck’s constant h. The Compton effect has also been written in a lucid manner involving the mathematical formulation from which the Compton shift can easily be obtained. This chapter also details the atomic hydrogen spectrum including Bohr’s model of an atom, energy of electron revolving around the nucleus, radius of an orbit and expression for velocity of electron. It also mentions the drawbacks of Bohr’s theory. References, problems, and questions on this concept are provided at the end of the chapter.