The Danish physicist Neils Bohr wanted to find out the structure of atoms. Ernest Rutherford had shown that atoms had a tiny central nucleus surrounded by even tinier, orbiting electrons. But there was something about this idea that Bohr found difficult to accept. The electrons should slowly loose their energy and spiral into the nucleus. Given enough time, all atoms should collapse. But their was no indication that this happened. Bohr wanted to look inside atoms to see what was going on. He needed to find a window, and 1913 he found one;
Neils Bohr and a window that echoes the orbital patterns he found in atoms, and the colours that they can produce
The secret can be seen in a plasma ball. We have one in school. Have a go at looking at it using your home made spectroscope.
Instead of a rainbow of colours you will see distinct lines of coloured light. In 1885 a Swiss school teacher called Johann Balmer looked at the light given off by a device similar to a plasma ball. His was filled with pure, low pressure hydrogen gas.
Johann Balmer
Putting the light from a Plasma ball through a prism (your spectroscope does it with a piece of CD) reveals emission lines of light.
Balmer measured the wavelengths of the coloured lines produced and was able to formulate a precise mathematical relationship between each of them.
Bohr took Kirchhoff's findings about hot gases being able to absorb and emit the same precise wavelengths of light, and combined them Balmer's mathematical relationships for hydrogen emission lines . He then matched them to the laws of physics and produced a model for the structure of atoms. His model was able to predict the exact energy levels of other orbits.
Bohr's model is a bit like an onion. Electrons are only found where the layers of the onion meet. They can jump from one layer to another but they cannot stay in between. To jump up to a higher level they need a precise amount of energy, no more and no less. They can get this energy from EM waves. For example the electrons in sodium atoms in the atmosphere of the Sun absorb a particular wavelength of yellow light to jump up to a higher level and a slightly different wavelength of yellow light to jump up to a neighbouring level. When they do this they leave their light fingerprints in the Sun's spectrum first noticed by Fraunhofer.
When you look at a candle through your home-made spectroscope the same electrons of the sodium atoms get energy from the atoms bashing together in the hot flame and are bounced up to the higher energy levels. They then fall back down and when they do they give out the two wavelengths of yellow light.
