 |
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!
