Why glass is transparent and other things not

[*Aqua*]

Hello everybody!

I saw

in which a professor explains why glass is transparent.

(Explanation in a nutshell.)

He says the photons with visible wavelength don't have enough energy to excite the electrons to a higher energy state. That's why they are not absorbed and simply wander through glass.

In colored glass some photons with a specific wavelength have enough energy to excite the electrons. They are therefore absorbed while photons of other wavelengths pass. The glass gets its color.

My question is the following:

Electrons are only allowed to have specific energy levels. For example it can have a low energy state and a high energy state - but not in between! The difference between the states is called energy gap. Let's assume a photon has 1.5x the energy of the energy gap. It is absorbed by a electron which takes 2/3 of the energy away.

What happens with the remaining 1/3? Will the electron emit a new photon with this left-over energy? Will it be converted to heat? A combination of both?

Sorry for the awkward English. It's not my native tongue.

Edited December 17, 2014 by *Aqua*

[K^2]

Lets start with the fact that that guy is saying horribly wrong things. Brick is made up of minerals, most of which are transparent in their crystalline form. In fact, most everything is at least somewhat transparent as a crystal. Or as a liquid. Or as a gas. It takes some effort to make something with homogeneous structure opaque.

And the reason for that sort of goes back to your question. If photon has too much energy, it can't be absorbed either. Glass is basically opaque in infrared, for example. You'd think if visible light has not enough energy to excite electrons in glass, IR wouldn't have a chance. But there are levels there that can be excited. Visible light simply has too much energy.

So there are really two ways that something can be opaque. Absorption is one of them, but material has to absorb across a broad range. A lot of organic materials with complex molecules do that. Metals also do that, because they have entire bands of energy levels accessible to electrons. But most non-conducting crystals do not. They might have absorption in a specific band of visible light, giving them a color, but they are still mostly transparent.

The second way is with refraction and diffraction of light. That's the real reason why bricks are not transparent. It's the reason sand isn't transparent. It's the reason that snow and crushed glass are not transparent. If you have a tiny little crystal of transparent material, instead of simply letting light through, it will be diffusing light. Take a lot of these together, and light simply cannot propagate in a straight line. What little light gets to the other side by random chance has to travel a much, much longer distance to get there, which means there are far more chances to get absorbed by some impurity. As a result, material is opaque.

[*Aqua*]

If photon has too much energy, it can't be absorbed either.

Ah! Now I understand!

So the photon's energy has to exactly match the energy gap to get absorbed.

What happens to the excited electron? I remember it will try to release energy to go back at its former energy state after some time. And if I recall correctly it will do that by emitting a photon. But that means it only delayed the light in passing the material, doesn't it? Ok it will be deflected several times as it wanders through the material and that will take some time, too. Is it possible to detect that light after the light source is turned off?

[K^2]

What happens to the excited electron? I remember it will try to release energy to go back at its former energy state after some time. And if I recall correctly it will do that by emitting a photon. But that means it only delayed the light in passing the material, doesn't it? Ok it will be deflected several times as it wanders through the material and that will take some time, too. Is it possible to detect that light after the light source is turned off?

It, typically, won't let go of that energy all in one go. There are usually several levels with lower energy bellow that one. In that case, instead of emitting one photon with all that energy, it will emit several with less energy. This will keep happening until energy levels are low enough to start exciting molecular or atomic vibrations. At that point, that energy is just heat.

[*Aqua*]

Thank you!

I think I now understand how all that works.

[andrewas]

Is it possible to detect that light after the light source is turned off?

Yes. This is how photoluminescence works - photons excite electrons in a substance chosen for a long decay time and an emission frequency in the visible range, and the object glows for a few minutes or hours as the electrons decay back to their original state.

[lajoswinkler]

Yes. This is how photoluminescence works - photons excite electrons in a substance chosen for a long decay time and an emission frequency in the visible range, and the object glows for a few minutes or hours as the electrons decay back to their original state.

That would be phosphorescence. Fluorescence deals with nanosecond range emission times. They are distinct processes.