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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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Nature’s hidden beauty – A tangent from Intro Bio

Photosynthesis is a way that nature observes the first law of thermodynamics.

As we all learn in school, the sun is the primary source of energy on Earth, but only a fraction of Earth’s residents can tap into that energy directly. The rest of us, the heterotrophs (from hetero- other and troph – food), get our energy indirectly. We either eat the plants (or other organisms) that produce their own food, or we eat the things that somewhere down the line got their energy from eating autotrophs (from auto- self).

But, because the first law of thermodynamics states that energy cannot be created or destroyed, but only converted from one form to another, these autotrophs could not make their food from nothing. Instead, they converted (solar) energy from the sun into chemical energy via photosynthesis.

Solar energy, which comes to Earth as photons, has characteristics of both particles and waves (as it turns out everything does). These waves have energy that is inversely proportional to the wavelength of the light- shorter wavelengths transfer more energy than longer ones. I like to think of it this way: Shorter wavelengths mean more waves per unit time. If you were one the beach watching waves come in to shore, if more waves crash on the beach in an hour on Saturday than on Sunday, then more energy was transferred per hour from the ocean waves to the shore on Saturday.

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Absorption spectrum of pigments

The visible light we can see only a small slice of the broader electromagnetic spectrum. Because we see only the light that bounces of things, if those things absorb some of that light (such as plants that use the light for photosynthesis), then we see only what they reflect back because it is not absorbed. This explains precisely why most leaves appear green – all but the green light is absorbed by pigment molecules that are collecting energy in the chloroplasts.

We can see this clearly by looking at an absorption spectrum of several pigments found in leaves.

What’s really interesting, is the beauty of flowers. These parts of the plant are not photosynthetic*, but they also contain pigment molecules. Why?

Of course we know this. Flowers are the reproductive organs of plants, and they often require assistance from insects or other animals for pollination. The way they attract pollinators is by giving a reward (nectar) and providing visual cues about where that reward can be found (the colorful flower).

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Visual spectrum comparison

But, it turns out that bees (a common pollinator) don’t see the same visual spectrum as we humans do. Instead, their spectrum is shifted slightly in the ultraviolet direction.

Naturally, this would have consequences. If bees can see UV light, it would be reasonable to expect that some flowers use pigments that make them visible at UV wavelengths. In fact, this is exactly what we see – well, what we would see if we could see UV. Here’s a representative flower shown as we see it and as a bee may see it – with a UV colored landing area right where the pollen and nectar are found.

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Natural Light / UV Light

*At least I think they aren’t. If anyone can provide an example of flower petals that photosynthesize, that would be greatly appreciated.

 
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Posted by on September 28, 2012 in Uncategorized

 

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