Atomic Physics in Physics

Atomic physics begins with something you can see in a laboratory as plain as day. Pass the light from a hydrogen discharge tube through a prism and it does not spread into a continuous rainbow, it breaks into a few sharp coloured lines, always the same ones. Nothing in classical physics explains why an atom should refuse to radiate at any other wavelength. Bohr's answer was to allow only certain orbits, which then reproduced the whole hydrogen spectrum from a single formula and the constant that goes with it. The blocks give that spectrum and the energies behind it, the Bohr relations for orbit size and speed, then the fuller quantum picture in which the electron is described by four numbers, and finally the two effects that turn this knowledge into a laser.