Has the hi visibility clothing fad gotten out of hand? seriously every 2nd person i walk past seems to be wearing them for no apparent reason it is beyond ridiculous
In general, materials that are transparent (and colorless) lack any allowable transitions between atomic or molecular electronic energy states that have energy differences corresponding to the energies of visible light photons. Things that are transparent, but colored, have electronic transitions that can absorb photons of some energies, letting other energy photons pass through. The color we see is the result of the photons that are not absorbed. Things like metals that are completely opaque (at least in thick samples) have electrons (the conduction-band electrons) that have extremely closely spaced energy levels so closely spaced that one can consider them to be a continuum. Electrons in metals are therefore free to absorb photons over a very wide range of energies. (Actually, the electrons in the metal end up wiggling back and forth in response to the oscillating electric field of the photons. These wiggling electrons are undergoing an acceleration. Accelerated charges radiate photons, and, in fact, the electrons in the metal re-radiate photons when they are excited by incoming photons. That why metals are shiny/reflective.) It is also possible for a material to be opaque simply because of scattering. If a material is polycrystalline, and if there is a difference in refractive indices across the grain boundaries (either because there is a different phase along the grain boundaries, or if the material is optically anisotropic, i.e., has different refractive indices along different crystallographic directions, then only some of the light encountering the grain boundary interface will be transmitted across the interface; the rest will be reflected. If the sample is thick enough, the numerous scattering events that occur will attenuate the intensity of a light ray traversing the sample. Many ceramics are opaque because of this phenomenon, even though they are made of materials that would be transparent in single-crystal form (e.g., alumina, Al2O3).
I can give you an example of what not to do. After I had my daughter, the pastor of the church my mom goes to sent me a card that had a few scriptures in it followed by God thanks you for not having an abortion. I got about 3 different cards from people at my parents' church that said something along those lines. I had to ask my mom if that was the Christian way of telling unwed mothers congratulations on the baby. Seriously, those were awkward cards to get.
