Different lines: the spectroscope

 

This is what undergraduates
understood by spectroscope,
even in the 1960s.

In 1802, an English chemist named Wollaston noticed a number of black lines in the spectrum of the Sun. Joseph von Fraunhofer (1787 – 1826) saw the same lines in 1814 and mapped them in more detail. He found 570 lines, and named them, according to their prominence. These days, better instruments can detect thousands of Fraunhofer lines across the solar spectrum, and Fraunhofer’s D line can now be distinguished as three separate lines. The new and improved instruments are now usually called spectrographs or spectrometers, but they are still used to dissect and examine spectra. Newton would have given his eye teeth to access one of them.

Fraunhofer’s newly-discovered lines were regarded as gaps in the spectrum, but each line represented a subtraction from a continuous spectrum, the removal of a key wavelength. This mystery stood for more than 40 years before Kirchhoff and Bunsen sorted it when Kirchhoff saw a similarity: some of the ‘dark’ lines in the solar spectrum matched ‘bright’ lines of emission spectra.

Fraunhofer lines, wikimedia
curid=7003857

The terms ‘dark’ and ‘bright’ are relative: in actual fact, the dark lines are only dark in contrast to the rest of the spectrum, and may even be brighter in absolute terms than the visible lines of an emission spectrum. What happened next is best described in Kirchhoff’s own words:

While engaged in a research carried out by Bunsen and myself in common on the spectra of coloured flames, by which it became possible to recognise the qualitative composition of complicated mixtures from the appearance of their spectra in the flame of the blow pipe, I made some observations which give an unexpected explanation of the origin of the Fraunhofer lines and allow us to draw conclusions from them about the composition of the sun’s atmosphere and perhaps also that of the brighter fixed stars.

These lines were hard to see. In his Decline of Science in England (1830), Charles Babbage referred to the problems encountered by an untrained observer. The ‘Mr Herschel’ in the story was William Herschel’s son, who later became Sir John Herschel, a good friend of Babbage, who named one of his sons Herschel Babbage, who was later a minor explorer in Australia.

Conversing with Mr. Herschel on the dark lines seen in the solar spectrum by Fraunhofer, he inquired whether I had seen them; and on my replying in the negative, and expressing a great desire to see them, he mentioned the extreme difficulty he had had, even with Fraunhofer’s description in his hand and the long time which it had cost him in detecting them. My friend then added, “I will prepare the apparatus, and put you in such a position that they shall be visible, and yet you shall look for them and not find them: after which, while you remain in the same position, I will instruct you how to see them, and you shall see them, and not merely wonder you did not see them before, but you shall find it impossible to look at the spectrum without seeing them.”

Over time, the instruments improved, and by 1864, William Huggins took the spectrum of a nebula. Before long, Doppler shifts (get the book!) were being measured on photographs of spectra, and we were on the way to the notions of expanding universes, Big Bangs and much more.

William Ramsay studied chemistry in Germany under Robert Bunsen, and in 1894, tackled a problem Lord Rayleigh had found with nitrogen. When nitrogen is made chemically, it has one density, when it is prepared by subtracting the other known gases from an air sample, it is slightly more dense. Ramsay remembered that Henry Cavendish had seen the same problem a century earlier, when he tried to combine all of the nitrogen in air with oxygen, but found there was always a bubble of gas left over. Ramsay heated gas with magnesium to make magnesium nitride, but still found a bubble of gas left behind, which was more dense than nitrogen.

Ramsay and Rayleigh had access to the spectroscope that Bunsen and Kirchhoff had introduced, and this revealed a spectrum which fitted no known element. They named the element ‘argon’, meaning ‘inert’. But, they reasoned, if there was one new element to fit into the periodic table (chapter 6), there should be more, one for each row of the table. Ramsay began the search, and looked at a sample of gas from a uranium mineral, cleveite, and found that the spectrum was that of a ‘metallic element’ previously discovered in the sun’s spectrum by Norman Lockyer, who had named it helium.

But what were the lines? The best way to answer this is to first go sideways for a bit. Glass is mainly sodium silicate, and no chemist who has ever heated glass in the flame of a Bunsen burner would doubt the sodium part. Like common salt, glass gives what looks like a distinctive yellow colour to the flame. We know now that there are actually two colours, with wavelengths of 589.592 and 588.995 nanometres, but for now, we can treat them as a single colour.

Fraunhofer’s newly-discovered lines represented a subtraction from a continuous spectrum, the removal of a key wavelength. If you view light that had passed through a medium rich in sodium, the ‘sodium colours’ are absorbed, leaving a ‘line’. As we understand it today, sodium ions in the flame absorb energy of that wavelength. The energy shifts an electron from a lower-energy orbital to a higher-energy orbital, and according to some ideas that we will look at later, that quantum, that very precise packet of energy, the difference between the two orbitals, is associated with a particular wavelength and colour.

If light passes through a cloud of sodium ions, light of that frequency will be extracted and used to ‘excite’ electrons. Later, the electrons drop back down to a lower energy level, and emit light of exactly the same frequency, but most of it goes sideways, so we miss seeing it in the light coming our way. Kirchhoff then described other similar experiments in which flames ‘doped’ with either sodium or lithium act as either absorbers or emitters on limelight and sunlight.

I conclude from these observations that a coloured flame in whose spectrum bright sharp lines appear so weakens rays of the colour of these lines, if they pass through it, that dark lines appear in place of the bright ones, whenever a source of light of sufficient intensity, in whose spectrum these lines are otherwise absent, is brought behind the flame.
— Monatsberichte der Akademie der Wissenschaft zu Berlin, October 1859.

Later, Anders Ångström would use spectroscopy to show there was hydrogen in the sun, Johann Balmer would explain the lines, and Norman Lockyer would find helium there as well, while William Crookes detected thallium without ever seeing it, by finding a green line in a spectrum from some residues in a sulfuric acid factory.

Jean Foucault, the inventor of Foucault’s pendulum (chapter 13 in the book), first discovered the way the emission and absorption effects are linked, but he failed to follow this through to a logical conclusion. Instead, it was left to Bunsen and Kirchhoff to reveal this discovery. And of course Bunsen and Kirchhoff used the heat of the Bunsen burner for their observations, but there was more to come, as readers of my book can see in chapter 10.

It cannot therefore be doubted that the extensive volcanic elevations constituting the high table-land of Armenia and the island Iceland have flowed from sources which were chemically identical… the mineralogical differences between those Caucasian and Icelandic rocks which present the same mean composition, are not less marked than those observed among other ferruginous rocks of plutonic origin.
— Robert Wilhelm Bunsen, Poggendorff’s Annalen, 1851, Scientific Memoirs, edited by Tyndall and Francis, 1853.

Now that was a difference!