Unit 15a: "Modern" Quantum Mechanics

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Question: why do some things emit a spectum of EM energy and others are just a few quantitized bands?

Blackbody is macroscopic, composed of a large number of atoms. It is well known from band theory of solids that when atoms combine to form a solid, their energy levels split and the separation between these energy levels is finer when the number of atoms is larger. The typical number of atoms in a solid is of the order of 10^23. Therefore, the separation between energy levels is very fine and we have a 'quasi-continuous' energy band. Whenever a blackbody emits radiation, it corresponds to an electron jumping from higher energy state to a lower energy state and because of this an electron in excited state has a very large number of 'energy levels below', it can jump into any of the empty states giving a continuous spectrum.  Likewise it is assumed Blackbodies can absorb all EM energies in the spectrum.

Quantitized energy emission and absorption would occur for single atoms (gases and plasmas)