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$s$ is sharp, $p$ for principal, $d$ for diffuse, $f$ for fundamental.

Where do all those term come from? I do not see any link with the corresponding shapes.

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This might help: "The letters, "s", "p", "d", and "f", for the first four values of l were chosen to be the first letters of properties of the spectral series observed in alkali metals." So, nothing to do with orbital shapes. en.wikipedia.org/wiki/… – Pygmalion May 21 '12 at 13:20
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The letters and words sharp, principal, diffuse, and fundamental refer to the visual impression left by the fine structure of the spectral lines which occurs due to the first relativistic corrections, especially the spin-orbital interaction. Sharp lines are sharp because $L=0$ and there's no degeneracy coming from here. Principal are principal because the overall strength of the lines is highest. Diffuse lines with $L=2$ look diffuse and fundamental $L=3$ lines are similar to the same lines in the Hydrogen atom, when it comes to frequency ratios. The terminology is obsolete and historical. – Luboš Motl May 21 '12 at 15:06
@LubošMotl This is an interesting question, and you have neat answer. Would you please put your comment as an answer, and elaborate a little more? Thanks! – QuantumDot Aug 23 '12 at 4:02

1 Answer

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Originally, the names were descriptions of series of spectral lines recorded on photographic film.

Introduction to Modern Optics By Grant R. Fowles

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This. it doesn't make sense in a textbook, but with the right kit can be reasonable in the lab. It's also a good time to introduce the idea of phenomenology as one of the ways to do physics. – dmckee♦ May 21 '12 at 13:59
If I remember correctly, the introduction of The theory of atomic spectra, by Condon and Shortley, has a good historical explanation as well. – Emilio Pisanty Nov 15 '12 at 1:04

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