Just curious as to what theories exist for the explaining of why some matter is opaque and some is transparent. Is it because the fields of atomic particles are so dense that light can either be fully absorbed or reflected but not allowed to pass through? Does light pass through anything or does it just filter in through the gaps? These sorts of questions come to mind. How would one change a sheet of clear glass' atomic structure to make it opaque? I am sure there are many other relevant questions regarding this subject. Thought it might be an interesting question. Please Register or Log in to view the hidden image!
Hi QQ, Just one thought on this topic. You can't consider something simply "transparent", but you should think specifically of transparency wrt a particular wavelength. E.g. water is transparent to visible light, but rather opaque to microwaves. That is why we see in the visible spectrum and cook food with microwaves. -Dale
Hi QQ, In short, the best existing theory of the interaction of light and matter is called quantum electrodynamics. However, for answering everday questions about opacity and so forth, simpler versions of the theory are often adequate. The simplest version, which is completely classical, consists of Newton's laws plus Maxwell's equations. A common middle of the road approach consists of treating matter quantum mechanically while continuing to describe light classically (Maxwell's equations). Of course, while the theory is all there, the application of it to a particular system can be complicated for any number of reasons. Suffice it to say that whole books can be (and have been) written about the optical properties of even a single material. Let me describe some examples, and you can let me know what you would like to have more information about. Metals: The dominant optical response in many metals comes from the conduction electrons. For a certain range of frequencies, metals are generally highly reflective. Depending on the surface structure, the reflection can be very diffuse (dull) or highly specular (shiny). At higher frequencies, metals can become transparent! Alternatively, some metals absorb strongly at higher frequencies corresponding to electronic transitions. Insulators: Since there are no conduction electrons, the dominant optical response in insulators tends to come from phonons (lattice vibrations). Insulators are often transparent in the visible range, but defects in the crystal structure can often give them color. Like metals, their behavior can be very frequency dependent. For example, glass is transparent in the visible, but it is opaque in the UV. In the end, many many things can affect optical properties. The list includes things like surface roughness, crystal defects, bound electrons, conduction electrons, phonons, grain boundaries, temperature, pressure, static magnetic and electric fields, and light itself (as in nonlinear optics).
Let me also answer some of your questions more directly. The response of materials in the visible often has little directly to do with the lattice structure. The wavelength of visible light is too long ~ 500 nm to care much about what happens at the level of the lattice spacing ~ .5 nm. You typically need X-rays to begin to see the atomic structure. As a consequence, your picture of gaps won't be very useful for understanding the properties of materials in the visible. For example, materials which are opaque are sometimes called "optically dense", but this does not necessarily mean the material itself is physically dense. Along these same lines, changing the atomic arrangement of the silicon dioxide molecules in glass won't do much to the transparency. In fact, glass is an amorphous solid, so there isn't any sort of definite crystal structure to change in the first place.
Physics Monkey, thanks for your response. I guess for me it comes down to this: Scenario: I have two tumblers [ vessels for drinking water from ] One is made of glass and allows me to see it's contents and the other is made of stone ware and dis-allows the viewing of it's contents. Both tumblers weight approx. the same and seem to have the same structural strength. What is it that makes the atomic structure of one object transparent and the other reflective? How does an energy field or magnetic field or an intense gravity field generate opaqueness? I guess these question may be essentially unanswerable at this point in our scientific evolution but I tend to think that they lie at the heart of what matter or mass is.
