Line spectra

1. Line spectra We all like the 4th of July fireworks. But can yu tell me what makes all those beautiful colors?

2. It has well known for centuries that many salts cause flames to assume brilliant colors. But systematic scien- tific investigations of this phenomenon started only about 150 years ago. They were pioneered by a German chemist Robert Bunsen. He designed a spe- cial 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, star- ted systematic studies, and found that the co- lors “split” into even more colors.

3. Here you can see what Bunsen observed in his experiments.

4. 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?

5. 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

6. 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: 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)

7. 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…..

8. The wavelengths of all those series were found to satisfy a common equation: 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).

9. 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 con- sists 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.

10. Bohr's model of Hydrogen Atom Another animation Yet another one The best I found