(File image) A glass prism separates light into its color components much as rain and sunlight can form a rainbow.
Why Is Glass Transparent Even Though It Is Made of Sand?
Delicious Reddit

DECEMBER 06, 2010
April Holladay, HappyNews Columnist

Light is an electromagnetic wave that rips through space — interacting with everything it encounters. In space, light meets little. A dense material like, glass, however, slows light as the wave "decides" how to interact with the material. But decisions aren't easy and there are many. The quantum-mechanical rules light follows are rigid, allowing only discrete answers: "yes" or "no." The rules forbid "almosts."
Q: Why is glass transparent even though it is made of sand?
Abhishek, Bhiwani, India
A: The wave's electrical and magnetic fields oscillate back and forth, and when the light wave hits a pane of glass, the wave's oscillations cause charged particles (primarily electrons) within glass molecules to vibrate back and forth as the electrons 'try' to absorb the light. Can they do it? Well, it depends.
If the "light" is high-energy (deep ultraviolet, for instance), yes glass molecules can absorb light. Glass, therefore, blocks deep ultraviolet and glass is not transparent to those rays.
"Glass is almost perfectly opaque to deep ultraviolet, which is why it's hard to get a tan through a window," emails physicist Louis A. Bloomfield of the University of Virginia. "By deep ultraviolet, I mean UVB and UVC."
But if the 'light' is visible, then the answer is no. Pure glass cannot absorb visible light. Light merely slows (as glass molecules decide what energy contained in visible light, if any, matches an energy level of an atom in the glass molecules). In fact, the speed of light through glass is 66% of its speed through a vacuum.
A little background: Molecules are a group of atoms held together by electro-static forces. An atom is a speck of matter consisting of a dense positively-charged nucleus surrounded by a system of tiny negatively-charged electrons.
Visible light from the Sun is made of light waves with many colors (which we see displayed in a rainbow, for example). Each color has a particular frequency and that frequency has an energy level proportional to that frequency, says Bloomfield.
Now the electrons of an atom also have discrete energy levels. What has to match is an electron's and a color's energy levels. If any of the energy levels of the light matches any of the energy levels of the electron, the electron absorbs that energy and the glass heats up.
It turns out none of glass electron energy levels match any of the energy levels in the colors of visible light. So light passes through glass. Thus, sun-illuminated pure glass doesn't get hot while the opaque frame around it does.
The electron decision process is almost a scene from science fiction. "In effect, the charged particle "plays" with the photon of light, trying to see if it can absorb that photon," writes Louis A. Bloomfield in How Everything Works. "As it plays, the charged particle begins to shift into a new quantum state — a "virtual" state."
Trying to decide if it can absorb the light, the electron tries an energy state (actually a light-wave color, say purple) on for "size," so to speak.
The electron shifts to a higher energy level, say a state between the red circle and the orange one shown in the hydrogen-atom figure. The electron realizes it's in a new "virtual" state (matching the color purple) and wonders, "do I get to stay here?" No, it concludes as it consults its list of allowable states. I'm between two allowable states (shown by the red and orange circles) so my virtual "purple" state doesn't match. Oops. The electron quickly returns to its original state (yellow). It can't stay in virtual reality long. To do so would violate the rules of quantum mechanics.
As the electron returns to its original state, the electron reemits the photon it was playing with, says Bloomfield, unchanged. That's why glass is transparent — glass can't absorb any frequency (color) of visible light. So it ends up reemitting the same light.
Sand isn't transparent partly because it does absorb some light colors (tiny grains of basalt, for example, absorb all frequencies so basalt sand appears black). Also, sand (tiny quartz grains for instance) has "countless tiny surfaces" that reflect about 4% of the light trying to pass through those surfaces, says Bloomfield. The small surfaces also bend the light as it exits the grain's surface so the light then travels in a different direction. The net result is that the grain doesn't reemit the light unchanged. Instead the grain's surface scatters light, so those sand grains appear white. Similarly, clear, crushed glass dust appears white.
I am indebted to physicist Louis A. Bloomfield of the University of Virginia for his discussion of how light passes through matter.
More exploring
Where does an atom get its energy? WonderQuest, February 2006
Further Reading:
How Everything Works: Making Physics Out of the Ordinary by Louis A. Bloomfield, Wiley (April 21, 2006)
Speed of Light in Transparent Materials, Interactive Java tutorial by Michael W. Davidson, Thursday, Jun 15, 2006