The first thing you need to know is what determines if an object is transparent (transmits visible light) or not.
Visible light is part of the elctromagnetic spectrum so this means it has some inherant energy. A convinient way of measuring that energy is with a unit called the electron-volt or eV. An electron volt is the energy gained by an electron accelerating through a potential difference of 1 volt. Visible light is the part of the electromagnetic spectrum between 1.7 and 3.1 eV.
In order for something to be transparent to visible light, the atoms in the material must not interfere with the transmission of that energy. This interference is most often accomplished when the material absorbs the energy from the light and uses it to excite one of it's electrons to a higher energy level.
The determining factor in if the material will absorb the light is a property known as the energy gap (Eg). Basically this is the amount of energy needed to excite a valence electron (an electron on the outermost unfilled shell of an atom) to the conduction state. Einstein showed that energy can only be absorbed in discrete packets called quanta (this is where the term quantum mechanics comes from), which means there will be some set value of energy at which point the material will start absorbing all energies equal to or greater than that value (again, this is called the optical gap).
This leads us to why glass is transparent.
Glass is a non-crystalline material, which means rather than having a regular and repeated alignment of its atoms it has a disordered random atomic structure. This is accomplished by cooling melted glass before the atoms can arrange themselves in a nice, neat way. In essence you must freeze-in the disordered liquid atomic structure. So glass is a non-crystalline solid and because of this, it's bonding is very strong and directional (highly covalent bonding). Since it is electrons that will absorb incoming light energy, there must be electrons available to do this. In glass there are very few free electrons for energy absorption so the optical gap is very high (greater than 3.1 eV). As a result light of energy less than the optical gap (in this case visible light) will pass through. It should be noted this is why glass is also a good electrical insulator.
And since I'm sure this will come up in your science fair on glass I'll answer another question too. Glass does not flow with time. There are some people that will claim because glass has a ';frozen-in'; liquid structure that with time it will slowly flow like a liquid, but this is not true. If glass could flow that way it would mean the bonding was very weak - and as you've just found out that would mean it couldn't be transparent!
Good luck with the science fair!Why is glass clear?
What about glass ';creep'; as stated by one of my professors, particularly old glass windows; measue to be thicker on the bottom than the top after a hundred years or so.
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glass is a silicate, and it allows all spectrums of white light to travel through it, when you tint glass to make it appear as a color, you are only letting either that color come through with the rest being asorbed, or letting that color only being reflected off as the others are asorbed. the color white reflects all colors of the spectrum back. the color black asorbs all colors giving it no reflection. clear allows light neither to reflect or asorb, but allows it mearly to pass through with possible bending of its path, which is why stuff looks distorted through glass when it is bent
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