In 1913, Niels Bohr explained why the lines in the hydrogen spectrum are arranged the way they are. His explanation places electrons in stable orbits which he called “stationary states” (violating well-known laws of science) and explains spectrum lines as the energy difference resulting from the movement of an electron from a higher-energy stationary state to a lower-energy one.
This moment not only ended almost 250 years of since the discovery of the spectrum, but was the beginning of fruitful ideas from one of the most profound intellects this world has ever seen. (L. Brillouin tells that he had entered the room just as Sommerfeld finished Bohr’s article in the July 1913 issue of Philosophical Magazine. Sommerfeld said “There is an extremely important paper here by N. Bohr which will be a milestone in theoretical physics.”) Bohr’s work continues to have an impact on science today.
The best place to start, I suppose, is at the beginning.
Matter and energy interact. Take a lump of metal and heat it. It glows, giving off light. Burn some wood and one of the things it emittes is light. Place an object in the sunlight and it gets hotter. Look at the sky after a rain and see a rainbow.
These things were known to the ancients. Today, we know that the energy given off by matter creates a spectrum. This spectrum and the behavior of its colors are so important to us that we have given its study a name — spectroscopy.