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Cosmos – on the nature of light

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.

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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.

ImageWe 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:

  1. continuous spectrum – emitted by a dense hot object
  2. 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)
  3. continuous spectrum with absorption lines – the inverse of the emission line spectrum. When a cooler gas absorbs wavelengths of light from a hot source.

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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 (*).

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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:

  1. for a good explanation of spectra http://www.astro.washington.edu/users/anamunn/Astro101/Project1/stellar_spectroscopy_introduction.html
  2. for the periodic table of light http://www.alexpetty.com/index.php/2011/07/20/the-periodic-table-of-the-light/
  3. 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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Posted by on April 7, 2014 in Uncategorized

 

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V GER

ImageLast week’s Science magazine had a pair of articles about Voyager 1’s arrival at the heliopause and the fluctuations of particles it has encountered. For about a year astronomers have been talking about the limits of the solar system.  An idea that I admit I had never entertained in any absolute way. Instead, my image of the extent of the solar system is mostly shaped by the most distant planets’ orbital paths. Sometime in the past decade or two, I became aware of plutoid objects, among which Pluto is one and that there is an Oort Cloud beyond that. I’ve always been a bit hazy about the details of what comprises the Oort cloud and how this differs from the plutoid objects.

Sometime during the conversation ignited by Pluto’s demotion to a dwarf planet, I head a good description of the solar system that described it as: four small, rocky planets close to the sun, then a belt of asteroids, followed by four large, gas planets, then another ring of small objects.

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The edge of the solar system

I really am looking for someone to explain this in terms that a reasonably intelligent person without much astronomy background can comprehend. That is, I don’t want too much left out, but I’m not necessarily ready for an overly technical explanation.

With respect to local suns, what is the position of our solar system? What forces interact between the suns? What do we know of the space between solar systems (or between galaxies?)

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A simplistic view of nine balls circling a star

That’s a pretty tidy description, but I think it leaves out a lot.

The interest now is in defining the edges, the limitation of the sun’s influence on space in favor of extra-solar forces. From what I gather, this is referring to both ‘solar wind’ and magnetic field.

I can understand this from the inside (although I need correcting here too) ,  but what I don’t fully grasp are what the forces are outside of the solar system. What dominates those forces? One thing I notice in the illustrations I’ve seen is a teardrop shape to the system resulting from a unidirectional current. What is this current? Is it emanating from other solar systems? Some local influence of nearby stars? Or a galactic force?

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Heliosphere warping under external pressure

I hope someone out there can help me understand this better.

 
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Posted by on July 20, 2013 in Uncategorized

 

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