In which I explain Kirchoff’s Laws of Spectroscopy

Kirchoff’s Laws are the laws that govern the behaviors of light passing through a gas.  They explain the behavior of the spectra of stars, require a basic understanding of quantum mechanics.  The powerpoint explains it with examples.

1.) A heated solid object produces a continuous (blackbody) spectrum.

This governs everything from stars to light bulb filaments to you. Yes, really; aside from the minute effects of human bioluminescence, people really do glow with thermal energy.  It’s just that human beings are cold enough that our blackbody curves peak in the mid-infrared, which we can’t see, but cameras can.  In the case of a light bulb, electrical charge is doing the heating; in the case of a star, nuclear fusion at the core is producing the energy (and the star isn’t really solid, but it more or less behaves that way)

2.) A heated gaseous object produces a bright lined spectrum.

The difference here is that the object is not solid, and thus all you are seeing are the spectral fingerprints of the atoms and molecules in the gas.  The specific fingerprint is unique to each chemical element and molecule, and is a product of its own electron orbitals.  Why?  Well, the heat is actually energy, and can be disposed of by emitting a photon.  However, as the electrons can only take on certain energy states (hence the “quanta” in “quantum”), they can only jump up or down specific amounts, releasing specific colors of photons.

3.) A heated solid surrounded by a gas produces a dark lined spectrum.

Now we have a gas, surrounded by a blackbody.  In this case (which essentially explains stars), the blackbody radiation passes through the gas, and the gas absorbs the same specific wavelengths of radiation they previously emitted.  Why aren’t they just re-emitted, filling in the absorption dips?  Well, for one thing, they are, just not necessarily in the same direction.  For another thing, energy can also be released by colliding with another atom (i.e., transferred back into heat) rather than emitting another photon.

Update: These slides are now citeable through Figshare!


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