Kirchoff's Laws

What do circuits, thermochemistry, and spectroscopy have in common? They were all advanced drastically by a man named Gustav Kirchoff. He has many laws named after him, but in this blog post, we will examine his three laws of spectroscopy.

Kirchoff's First Law of Spectroscopy: Dense objects (solids, liquids and very dense gasses like in the sun) will emit a continuous light spectrum if excited.
The consequence of this law is actually very important for our day to day lives. If the sun only emitted the wavelengths of light that Hydrogen and Helium atoms emit (those two elements make up a vast majority of the sun), we would live in a very different looking world. The only colors that we would see would be the colors shown below.

(http://keywordsuggest.org/gallery/87344.html)

Kirchoff's Second Law of Spectroscopy: Low density gasses will emit an emission spectrum (like the picture above) if excited by a high energy source.

Kirchoff's Third Law of Spectroscopy: Low density, cold gasses will absorb certain wavelengths from a full spectrum. When you think about this in the context of the Second Law, it makes sense. The wavelengths of light are exciting the cold gas, which then emits a photon of the same energy (same wavelength), but in a random direction. If you're looking at the source through this cold low density cloud, most of the photons of that wavelength have been scattered by the gas, and thus you see this: 

(http://www.astronomyknowhow.com/spectral-lines.htm)

One of the best ways of visualizing how these three laws work together is with the following diagram:

(https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect5/lecture5a.html)

As you can see, the blackbody (a term also coined by Kirchoff), emits a full spectrum because it is ultra dense. When the full spectrum passes through a cold, low-density cloud, the observer looking directly at the black body through the cloud sees missing parts of the spectrum. However, the observer viewing only the cloud from a different angle sees the emission lines from the excited atoms within the cloud.

These three laws have great importance in astronomy. Because the lines produced, whether absorption or emission, are unique to each element, astronomers can use spectra as methods of determining the makeup of lower density objects. This may not seem like all that big of a deal, after all, stars are very dense, and planets are dense, so all we can look at is clouds? Actually, the outer layers of stars are not very dense, and we can use this to see the makeup of the star. Additionally, planets have atmospheres, and we are coming upon an exciting time in the search for life and habitable planets as our technology nears the point where we can determine reliable spectra of the atmospheres of Earth-sized planets.