I have experimented with passing high voltage arcs through ionic compounds and have observed different colors when I do. An arc through salt (sodium chloride) produces a brilliant yellow light. How does this work? — JB, Lantana, FL
When electric current passes through air as an arc, the air becomes hot enough to vaporize the compounds you expose to it. As a result, there are individual sodium and chlorine atoms moving about in the arc itself. Like all atoms, a sodium atom resembles a tiny planetary system. It has 11 negatively charged electrons orbiting a massive, positively charged nucleus. But unlike our experience with the solar system, the electrons in a sodium atom can only travel in certain allowed orbits or “orbitals.” These electrons are normally found in the orbitals with the lowest possible energy. But when charged particles in the arc collide with sodium atoms, they often shift electrons in those atoms to orbitals with more energy. The electrons quickly return to their original orbits and emit their excess energies as light during their returns. In the case of sodium, the final step of the most common return path results in the emission of yellow light with a wavelength of about 590 nanometers. This yellow light is the same one you see in the sodium vapor lamps that are used to light highways and parking lots.
While sodium tends to emit yellow light, other atoms have different orbital structures and emit their own characteristic colors. Copper and barium atoms emit blue/green light while strontium atoms emit red light. These colored lights are the same ones that you see in fireworks.