© T.W. Hänsch.
This spectrum of atomic hydrogen was taken as an exercise by T. W Hänsch early in his career while he was pioneering laser spectroscopy. This group of spectral lines, known as the Balmer series, played a crucial role for Niels Bohr as he developed his model of the atom. Hänsch continued to refine hydrogen spectroscopy over the years and has obtained an absolute precision of a few parts in 1014. This research program also led to the invention of the optical frequency comb, which plays a critical role in the precise atomic clocks described in Section 8. (Unit: 5)