Spectroscopy
In this weekend’s Cosmos, a lot of attention was spent discussing the properties of light. For something so apparently simple, there is a lot beneath the surface.
I wanted to talk about two elements of this episode in particular and provide some examples to explain things a bit better.
The first idea is that white light (what we get from out sun) is composed of all the colors. What we see as colors is actually the various wavelengths of light. We see short wavelengths as colors toward the red end of the spectrum; longer wavelengths appear as colors toward the blue end.
We also know that shorted wavelengths carry more energy. I like to tell my students to imagine a shoreline where all the waves are exactly the same height. If the length of the wave is shorter (measure from the top of one wave to the top of the next), then more waves batter the shore per unit of time. Longer waves mean fewer waves hit the shore in a given period of time. So, is more energy transmitted to the shore from the longer or the shorter waves?
Another part of this ‘white light contains all wavelengths of light’ comes from the way a prism reflects and refracts light. Any wave will change its direction as it goes from one medium (like air) to another (like glass) – it actually changes speed, which suggests a good analogy that I’ll explain in a second. How much it bends depends on the wavelength of the light.
The analogy is that of a car driving on a street. Imagine the car veering of the street at an angle to the right. As it leaves the road, it hits mud. The right wheel hits the mud first and slows down pulling the car harder to the right until the left wheel hits the mud. When that happens, the car stops getting pulled to the right and goes off in a straight line again. (the moment when the car is getting pulled onto a new course I’ve drawn a dotted line) The important thing to note is that the car was pulled right by the icky mud clinging to the tires more than the road does.
We can bend light by passing it through a glass (prism). The result is depicted in this album cover for Pink Floyd’s Dark Side of the Moon.
We can even bring the colors back together to produce white light again by using a second prism.
All this gets us to the idea that light can be dissected into a spectrum using a prism. This is the first type of spectrum described below.
The three types of spectra:
- continuous spectrum – emitted by a dense hot object
- emission line spectrum – the precise wavelengths of light emitted from a hot gas (we can ignore this type of spectrum for the purpose of this discussion)
- continuous spectrum with absorption lines – the inverse of the emission line spectrum. When a cooler gas absorbs wavelengths of light from a hot source.
In the example discussed in Cosmos this weekend, we learned about the third spectrum. This is what is produced when a hot star emits light in a continuous spectrum. The cooler atmosphere of the star then absorbs some wavelengths of the light as it passes through. This is how DeGrass Tyson was saying that we could determine the composition of a star’s atmosphere from its spectrum. All we need is to do some experiments in the lab and see what absorption lines we see from different elements’ gas.
As always, the theory is cleaner than the reality, but let’s take a look at the spectrum from the sun. This image highlights some major bands and indicates which elements they come from.
Below the solar spectrum are some of the spectra from the sun’s constituents with major bands that correspond to those seen in the solar spectrum marked with (*).
Again, I apologize for this not being very exact, but it does at least communicate the idea of what was discussed on Cosmos in a little more detail.
References:
- for a good explanation of spectra http://www.astro.washington.edu/users/anamunn/Astro101/Project1/stellar_spectroscopy_introduction.html
- for the periodic table of light http://www.alexpetty.com/index.php/2011/07/20/the-periodic-table-of-the-light/
- for the composition of the sun http://chemistry.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=chemistry&cdn=education&tm=41&f=10&su=p284.13.342.ip_&tt=65&bt=0&bts=0&zu=http%3A//imagine.gsfc.nasa.gov/docs/ask_astro/answers/961112a.html
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