How does transparency work?

Why materials are transparent to some wavelengths and opaque to other? How does light pass through something anyway?

Basically speaking, opacity means that a fraction of photons in the visible range excite certain electrons in the atoms of the substance, and such an excitation results in absorbance and re-emission of the photons.  If the substance's atoms absorb enough of the photons, and/or involve enough frequencies in the stream of photons, then the photons will not reach the opposite side of the substance.

In reality, opacity is a range based upon intensity and frequency.  You can understand how, say, a color filter is a very selective absorber of photon frequency.  Also, you can understand that even a thin metal foil is a strong absorber of photon intensity.  There isn't a substance that is absolutely opaque, as there isn't a substance that is absolutely transparent.  It's all up to the atomic composition, and we have only ~90 types of atoms to work with, and largely ~20 of those are used in common materials where transparency would be tested.

Another way to understand this is to recognize that tossing light at an object engages its electrons.  If you're tossing a laser at a certain frequency into a material whose atomic structure doesn't engage with that frequency, then that object is largely transparent (noting that there will always be some scattering).  If you choose a material for that laser's frequency that has a strong excitation potential, then you should expect the object to be very opaque to the beam.  With other sources of photonic emission (not just visible light), other substances have varying degrees of opacity.  (A famous example of this is Earth's atmosphere, which is generally transparent to visible light, but is fairly opaque to infrared light ... which is how the "greenhouse effect" seems to function.)

>>2
It's not just atomic composition. Molecular structure influences absorption and reflection as well.

This takes us back to the question: Where does the coupling constant "e" come from?
Why is it always approx. -0.08542455 (or if you prefer, the inverse of its square, approx. 137.03597)?
It has been a mystery  ever since it was first discoveres more than 70 years ago.

>>4
It's alpha, fag.

>>5
Care to elaborate?

The Annex is fucking awesome.

http://annex.wikia.com

>>3
You're right, since molecules are formed from atomic bonds, which alters them by definition, hence alters their reaction to photons.

>>5
http://en.wikipedia.org/wiki/Fine_structure_constant

At least, I've only seen it referred to as alpha.  e almost always refer to the electron's charge.