Absorption and Emission Spectra 

It was a mystery of the highest order, baffling the greatest scientists of its day. It was a herald pointing to the heart of matter and a new way of seeing the entire physical world. 

In the last few decades of the 19th century, physicists and astronomers began exploring the spectra emitted by all kinds of terrestrial and cosmic objects. A spectrum is the amount of light given off by a glowing object (a burning match, a light bulb, a star) at different wavelengths. By examining the light created when different materials were burned in a laboratory, physicists discovered each element emitted its own very different, very distinctive spectrum. Instead of a continuous blend of colors (wavelengths), an element like hydrogen or helium would only emit light at specific wavelengths. Pass the light from the burning element through a prism and you would see a dark background and an irregular picket fence of bright lines (corresponding to wavelengths where light was emitted). Physicists called these patterns "emission-line spectra". 

At about the same time, astronomers were taking the light they caught with their telescopes and passing it through prisms to make their own spectra. When they looked at stars such as the Sun they found the opposite of what physicists saw in the lab. Instead of bright lines against a dark background, the astronomers saw a continuous rainbow of colors superimposed with dark lines. Since the dark lines meant light was being removed from the background rainbow, these were called "absorption spectra". To make the mystery deeper, the dark lines in the astronomical spectra often appeared at wavelengths that matched the bright lines of emission spectra. 

Below are examples of emission and absorbtion spectra for Hydrogen
 

Why were the spectra from each atom different? Why did some spectra consist of bright lines on a dark background while others consisted of dark lines on a bright background? Most scientists believed these elemental fingerprints were revealing clues to the nature of atoms (about which little was known at the time). No one, however, understood how to read these clues. Even the greatest minds of the age failed to create a workable model of the atom that could recover the funny pattern of emission and absorption lines. Then, along came an unknown 27-year old physics student who had the audacity to change everything.