Monday, April 25, 2011

How You See - Light Interacting with Matter

"There's this tremendous mess of waves all over in space, which is the light bouncing around the room and going from one thing to the other.  And it's all really there!  But you gotta stop and think about it--about the complexity to really get the pleasure."  --Richard Feynman 

I love to hear Feynman talk about the intellectual pleasure that contemplating reality can bring.  He actually has a book by the title of "The Pleasure of Finding Things Out".  With that taste and thirst for the pleasure of the true and real let's delve into what the light you're reading this blog is all about.  :)

First, we have to understand something about atoms to understand something about light.  (Before I proceed know that I am very open to being corrected!  I am not an expert on quantum electrodynamics.  I'm just someone who's curious and likes to share!  I could be wrong about this stuff and want you to let me know!)  As you know, electrons orbit around the nucleus of an atom within shells, they're 'quantized'.  Why?  Why not just any where they want to be?  (We could also ask why they don't just crash into the nucleus!)  Well, at least in analogy, because the areas in between the shells cause some what of an interference pattern that pushes the electrons back into these pockets of copacetic vibrations of waves.  It's a lot like harmonics on a guitar.





The dots you're seeing are points at which you can put your finger while it is vibrating and cause all other frequencies of vibrations to be canceled thus only leaving that pitch.  

Notice the nodes:


It's these shells that are vitally important to understanding how light interacts with matter and makes seeing possible.

How is light made?  One way is that energetic atoms slam into each other so hard that the electrons bounce around between shells (making a quantum leap) and this jostling emits energy in the form of photons.  How?  Well, when electrons go down an electron shell that lost energy just can't disappear so it gets emitted as a photon.  The bigger the drop the more energetic the photon.


So, how is it that we see things?  Meaning, how is it that light is reflected?  What happens when light hits material?

  • Reflection:  
    • Chew on this.  When you're outside during the day the light that you're seeing with traveled 93 million miles from a seething cauldron of energy created by atoms fusing.  The photon strikes an atom causing an electron to jump up a shell (multiple shells more likely) then that electron nearly instantly descends back down to ground state emitting another photon of equal or lesser value energy.  This is flawed way of thinking about it, but in my mind I almost see atoms breathing in photons (puffing up as its electrons rotate further out) and then breathing them out.
    • Metallic Substances:  Metals are special for a number of reasons; one of which is that they share electrons.  This being the case they easily jump up a shell when a photon hits them and then they are also easily able to drop back down.  Metal is shiny because those electrons just bounce right off that sea of electrons that are shared by the atoms. 
    • Florescence/Phosphorescence: A photon hits, pops an electron up a shell by exciting it, it then descends, but not as much as it was elevated and emits a lower energy wave (like neat neon glowing colors under a black light[which is UV]).  
Neat black light tattoos.

  • Absorption
    • Color: How do we explain things having different colors?  Mostly from absorption.  Say for example white light hits a green object.  It's green because all the other spectra are absorbed (yellow, red, blue, purple, etc) and only green light is reflected.  So, what happens to the other spectra?  If my understanding is correct it's this: a photon hits an object, an electron becomes excited and pops up a shell.  Instead of falling down and reflecting the same wave length light it slowly comes down slowly, intermediate shell by intermediate shell.  This very well may emit something of lesser energy like infrared that then warms up the material.  (Some of this is educated speculation since there's such a dearth of easily accessible in depth explanations out there.  Please help.)

    • Solar power - Photon hits a metal, an electron pops up to an elevated state and is then transfered as a current.


Transmittance
  • How does light go through material?  The photon hits an electron but does not have enough energy to get the electron up to another shell and therefore the photon keeps on moving in the same direction.  We still know that the photon is interacting with the electron, though, since the speed of light is slowed down through material (the speed of light is only constant in a vacuum).   Normally light travels at something like 671,000,000 miles per hour, but scientists have gotten light to go as slow as 38 miles per hour going through extremely cold super-atomic clouds.

  • Sun screen:  Is a great example of partial absorbance.  Visible light isn't high enough energy to get the electron to jump to the next shell (so it appears clear), but UV light is and is therefore absorbed!

  • Polarization: It's these interactions that can affect the orientation of the oscillation of the photons and cause them to get directed into one direction or spin.

  • Water color filtration:  Why is it that red light doesn't travel very far in water?   Why is water blue?  I still need clarification on this, but we're basically talking about specific partial absorbance.  If my understanding of light attenuation through water is correct, the photon isn't just interacting with the electron as a independent agent, but as a molecule.  Molecules have a natural vibrational frequency that light can harmonize with.  Something like this perhaps: a photon hits an electron, that photon causes the electron that is held in place by electro-static forces to vibrate the entire molecule.  The likelihood of the photon to strike the electron/molecule at the right place to resonate is what causes the partial absorbance.  This website explains the possible vibrations pretty well.


Please correct me where I am wrong!


Images from here, here, here, here, here, here, here, here, here, here, here, here and here.

Works sighted [sic]:


http://www.youtube.com/watch?v=2BnoZ71BO0U
http://www.madsci.org/posts/archives/1999-08/934381368.Ph.r.html
http://www.blurtit.com/q633182.html
https://docs.google.com/viewer?url=http%3A%2F%2Fwww.wsc.ma.edu%2Fcmasi%2Fgen_chem1%2FLecture_notes%2Fp126-132%2520Waves%2C%2520Photoelec%2C%2520.pdf
http://www.scientificamerican.com/article.cfm?id=how-do-mirrors-reflect-ph
http://www.andor.com/learning/light/?docid=333
http://en.wikipedia.org/wiki/Reflection_(physics)
http://en.wikipedia.org/wiki/Quantum_electrodynamics
http://www.theoryofabsolutes.com/photonreflectionindepth.html
http://www.vega.org.uk/video/subseries/8
http://www.youtube.com/watch?v=B7pACq_xWyw&feature=player_embedded#at=486
http://www.youtube.com/watch?v=-YYBCNQnYNM&feature=related
http://news.harvard.edu/gazette/1999/02.18/light.html
http://en.wikipedia.org/wiki/Attenuation
http://www.dartmouth.edu/~etrnsfer/water.htm
http://en.wikipedia.org/wiki/Polarization_(waves)#Biology
http://www.btinternet.com/~martin.chaplin/vibrat.html



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