Well, what does “transparent” mean? It means that any light being reflected in our direction from an object outside a glass window, for example, can pass right through the glass unobstructed and come out the other side, where our eyes can deal with it.
We therefore see the object through the window. That’s why people who have little regard for the English language use the term “see-thru” (invariably spelled that way) instead of the perfectly good word transparent.
In general, when a traveling ray of light encounters a new substance, it may be reflected backward from the surface or it may penetrate the surface and be absorbed. If it manages to escape both of these fates, it can continue traveling through the medium; it will be transmitted. So our job is to explain why air, water and glass don’t reflect and/or absorb very much of the light they receive. Almost all other substances, except some water-like liquids and glass-like plastics, absorb some of the light and reflect most of it, leaving practically none to be transmitted.
Let’s get air out of the way first. Under ordinary conditions, the spaces between air molecules are around ten times bigger than the molecules themselves. So air is almost completely empty space, containing virtually nothing that could interfere with the passage of light except for a very occasional molecule. Ditto for all gases.
Water and glass are quite a different ball game, however, because their molecules are very close together, close enough to do a fair amount of reflecting. Remember the glare from that pond’s surface or from that car’s windshield on a sunny day? So even from the most transparent liquid or solid substances, some light is reflected. It depends on the angle at which the light hits the surface.
Of the light rays that do succeed in penetrating air, water or glass, very, very few of them are absorbed; almost all the light gets through. Molecules absorb light because their electrons have certain preferred energies, and by taking on the extra energy of a light particle (a photon), they can reach another, higher one of their preferred energies.
It happens that none of the molecules in air, water or glass can absorb and “use” any of the energies in visible light; the energies that they can absorb are certain radiations that humans can’t see, such as ultraviolet and infrared radiations. Ditto for alcohol, kerosene and other familiar transparent liquids. So if very little light is absorbed and the angle isn’t right for reflecting, almost all of the light will go straight through by default.
There are, of course, colored glasses, liquids and even gases. What’s going on there is that they selectively absorb some of the wavelengths or energies in white (or colorless) light and transmit only those that they can’t “use.” The transmitted light therefore has a different composition of wavelengths from white light and hence a perceived color.
