THE FUNDAMENTALS OF PHYSICS
SCIENCEPHOTO^ LIBRARY,^ GETTY5 spectroscopy, they found that each
element produced bright lines in the
spectrum at precise wavelengths. The
lines occurred in the yellow part of
the spectrum for sodium, in the green/
blue pa r t of t he spect r um for copper,
and so on. One evening, from their
laborator y in Heidelberg, Bunsen a nd
Kirchhoff were able to analyse the
light from a major fire in Mannheim,
some 16km away and identify linesproduced by the presence of strontium
and barium in the blaze.
A few days later, the pair was taking
a break from the lab with a walk along
the Neckar River, which flows through
Heidelberg, discussing what t hey had
seen in the fire. According to legend,
Bunsen remarked to Kirchhoff
something along the lines of:
“If we can determine the nature of
substances burning in Mannheim,Copper being burnt
in a flame from a
Bunsen burner. Copper
burns with a green/
blue flame in the
presence of oxygen to
form copper (II) oxideBy studying the spectrum of the Sun, Rober t Bunsen and Gustav Kirchof f mastered
spectroscopy and could see for the first time the elements that make up our nearest starTHE KEY EXPERIMENT
After their observations of the fire in the city
of Mannheim, Bunsen and Kirchoff
wondered if they could analyse sunlight
in the same way. They looked at the lines
associated with sodium in flame tests, and
tried to find out if these bright lines exactly
matched the corresponding dark lines insunlight. To do this, they reflected sunlight
through the flame of a Bunsen burner, that
had been ‘doped’ with a little sodium, and
analysed its spectrum. They expected that if
both lines had exactly the same wavelength,
the dark solar line would be ‘filled in’ by the
bright sodium line. But to their surprise, theyfound that the dark line was made even
darker. Kirchoff quickly realised that sodium
in the flame was actually absorbing some
of the sunlight, and that therefore these
particular dark lines in the solar spectrum
were being caused by sodium in the Sun’s
atmosphere absorbing light from below.
In that case, the other Fraunhofer lines
must be caused by other elements
absorbing light. Kirchoff’s discovery was
presented to the Prussian Academy of
Sciences in Berlin on 27 October 1859.The spectroscope that Bunsen and Kirchoff used to study sunlight. In doing so, they discovered some of the elements in the SunMAGNIFYING EYEPIECEPRISMPLATE WITH THIN SLITFLAME