Glossary / History of physics
Fine structure
The splitting of what looks like a single line in an atom's spectrum into several closely spaced lines when examined in detail.
In 1916 Arnold Sommerfeld extended Bohr's circular orbits to ellipses and brought relativity into the electron's motion, showing that the energy shifts slightly with the orbit's shape and splits the lines[1]. The pattern matched measurements well.
The number of about 1/137 that appeared is the fine-structure constant, still central to physics as the measure of the strength of electromagnetism. It later turned out that the agreement was partly lucky: the correct theory, including electron spin, happens to give the same answer.
Related terms
- Bohr modelThe 1913 model in which electrons circle only on certain orbits and emit light of definite energy when jumping between them.
- Zeeman effectThe splitting of spectral lines into several when atoms are placed in a magnetic field.
- Old quantum theoryThe theories of roughly 1900 to 1925, from Planck onwards, that explained phenomena by adding quantum conditions on top of classical physics.