Transcript Slide 1

Line spectra
We all like the 4th of July
fireworks. But can yu tell
me what makes all those beautiful colors?
It has well known for centuries that many salts cause
flames to assume brilliant colors. But systematic scientific investigations of this phenomenon started only
about 150 years ago. They were pioneered by a
German chemist Robert Bunsen. He designed a special gas burner, and inserted into the flame samples
of various salts to the on a piece of platinum wire:
But Bunsen was a real
Scientist. Just admiring the
colors was not enough for
him! He used a prism, started systematic studies,
and found
that the colors “split”
into even more colors.
Here you can see what Bunsen observed in his
experiments.
Bunsen established that each element
produces a unique pattern of lines. It was
How a technique known as “spectral analysis”
emerged. The “barcode-like” spectra can be
thought of as “characteristic signatures” of
elements. Spectral analysis makes it possible
to detect even a tiny amount of an element in
a mixture of many others.
However, for a long time it was not known why
the spectra were so different. In fact, there
was even no explanation why the spectra
consist of a number of narrow lines. Why not
a continuous spectrum, like the one from a
blackbody?
Soon the Bunsen burner was replaced
by something better – the “Geissler tube”
The tube was invented by the German
physicist and glassblower Heinrich Geissler
in 1857. The Geissler tube was an evacuated
glass cylinder with an electrode at each end.
A Geissler tube contain one or more of the
following: rarefied (thinned) gasses such as
neon, argon, or air. When a high voltage is
applied to the terminals an electrical current
flows through the tube, and the gas inside
starts glowing.
Next step after Bunsen burner
First step toward understanding the spectra
In 1885, a Swiss schoolteacher Johann Balmer
examined the line spectrum of hydrogen and
found that it fit the simple equation:
 n2 
 withn  3,4,5,
  364.5nm  2
n 4
However, Balmer studied only the visible part
of the spectrum. Other series were discovered
when people started investigating the regions
of shorter wavelengths (the “ultraviolet”, or UV)
and longer wavelengths (the “infrared”, or IR)
Soon other series were discovered: Lyman
series in UV:
and several series in the infrared region,
named after people wh discovered them:
Paschen, Brackett, Pfund…..
The wavelengths of all those series were found
to satisfy a common equation:
n2
  min 2 2
n  n0
n  n0 
where min and n0 are differentfor each series.
The work of these gentlemen showed that there
Is a clear regularity in the wavelengths emitted.
Still, the reason why it was so remained a puzzle.
Efforts of finding analogous regularities for other
element all failed (it was perhaps one of the main
reason why attention focused on hydrogen –people
had a right instinct telling them that hydrogen is
the “key” that will eventually solve the puzzle).
A event that supplied crucial info needed for
Understanding the hydrogen spectrum was the
discovery of the atomic nucleus by Rutherford.
The Rutherford Experiment
The electron had been discovered a few years
earlier, so now it became clear that matter consists of positively charged nuclei, in which about
99.95%o of all mass is contained, and negatively
charged electrons.
It was a “flash of intuition” that led then a Danish
Physicist, Niels Bohr, to develop a model based
on the analogy with a planetary system.
Bohr's model of Hydrogen Atom
Another animation
Yet another one
The best I found