File:Quantum Mechanical Spin Orbit Coupling in a candle flame.jpg

English: Spectrum shown in this picture consist of two discrete yellow emission lines over a broad continuum spectrum of colors. Continuum spectrum originates from the upper part of the wax candle flame which approximates a black body emission. Discrete lines come from the excited sodium atoms (Sodium and chlorine bond broken and atoms are excited to higher energy states by heat energy from the candle) from a kitchen salt, as they relax to ground state emitting discrete wavelengths of light at ~ 589 nm and 589.6 nm.

For several years, existence of such doublets, two closely spaced lines instead of one discrete line in many Alkali atoms spectra was a mystery as traditional quantum mechanics was unable to explain this observation. Further investigations (such as Stern-Galrach experiment) confirmed that this splitting is cause due to the fact that electron, along with a charged particle, is also a tiny magnet. Electron's intrinsic magnetic field interact with the orbital magnetic field produced by electron motion around the atomic nucleus resulting in splitting of energy levels. This is also known as spin-orbit coupling since in quantum mechanics, magnetic property of electron is known as 'spin' .Spin can have only two discrete values +1/2 and -1/2.

This picture was taken using a common wax candle with some kitchen salt added in the flame in order to obtain the sodium doublet spectrum. A spectroscope with a 1000 lines/mm transmission grating was used to split the light into its component colors and Canon T3i DSLR camera was used as a detector.