[londounkm (OP)]

I was pondering something today and not being of a particularly scientific nature i wonder if someone out there may be able to help. I will state what i understand to be true, please feel free to correct and adjust what i am saying and hopefully i can come to understand the whole concept.

I was trying to understand what the concept of colour is, and how that relates to the elements and objects we see around us. As I understand it, colour is the by product of how Photons interact with a given thing as well as the perspective of the person or thing that is viewing it. Lets assume that i have perfect vision for the purposes of this conversation.

So i have a dish in front of me that contains one gramme of pure gold, lucky me!! The light photons hit the gold atoms, and interact with the electrons. Some might pass through the mass, some might be refracted away from my eyes but some will hit the electrons and then will be returned to my eyes. The result of this process is that i see an object that is Gold in colour.

I guess my first question is why i see any colour at all and the one gramme of gold is not just see through. As i understand it there are no solid elements from the periodic table of elements that are see through. I imagine that this is because the photons are not able to simply pass through the gold atom unaltered. Some are absorbed, some pass through and some are reflected, the photons that are reflected are interpreted as gold by my eyes.

Do the photons interact with the Electrons, Nucleus or Both? I am assuming that the electrons have very little to do with the colour that we see as if we were to change the charge of the one gramme of gold atoms from neutral and therefore change the amount of electrons per neutral atom from 79, the actual colour of the gramme of gold remains the same. This leads me to the conclusion that the aspect of colour must be determined by the interaction of the photons with the nucleus of the atom.

From my basic knowledge of Chemistry the Nucleus is comprised of many smaller components but can be generalised as Protons and Neutrons. In Gold we have 79 Protons, and if the number of protons were changed then the atom would no longer be gold. The number of Neutrons in Gold is 118 (∑(197-79)), and the number of Neutrons can change in the atom without creating a new element, instead this just creates a new isotope of Gold. To the best of my knowledge changing the number of neutrons would not alter the colour of Gold. So does this mean that the colour of Gold, has a direct relationship to the fact that it has 79 Protons, and it is the interaction of light Photons with the 79 protons that leads to me seeing Gold as a gold colour?

Finally, I know that Glass is normally made of Silicon and Oxygen and some types of glass have traces of boron, calcium and sodium. How is it that Silicon in its normal state is not see through but when combined with Oxygen it suddenly is? Is it because on its own Silicon would has 14 protons but when combined with Oxygen this results in SiO2 which has 30 Protons?

Well i think i have spouted just about enough rubbish to give you an idea at what i am getting at and also where my mind is currently at so that someone can hopefully help me out and point me in the right direction.

[another_someone]

OK, firstly, visible light only reacts with the electrons of the atoms.

Colour can have two meanings.  On a spectroscope, it is the actual frequency of light in the spectrum, but in ordinary human terms, we are talking about perceived colour.  Perceived colour depends on how the light interacts with the cells in the retina of your eye, and those cells only give an approximation of colour (this is why you can mix red and blue and appear to get magenta, but a spectroscope will still see red and blue, but your eye will see magenta).

Because light interacts with the electrons in a material, and the state of those electrons change when they form chemical compounds, so the colour of compounds may be different to the colours of the elements they are made up from, because the chemical bonds in the compounds are changing the energies of the electrons, and so changing the energies of light to which those electrons will react.

As I said, perceived colour is a physiological process, and as such, different species of animals can differentiate different colours.  Two flowers which to the human eye may both seem to be white, might look very different colours to a bird or an insect, as many birds and insects can see into the ultra violet.  On the other hand, many other mammals can see fewer colours than humans can.

[londounkm (OP)]

thank you very much that really helps with explaining things a bit better.

Just to make sure i understand you correctly visible light only interacts with the electrons of atoms, very clear. So with glass for example, the fact that Silicon has been combined with Oxygen, this has changed the amount of electrons as well as the structure of the atom and it is this aspect of the new component, glass over the silicon on its own that allows the light to now pass through.

Oxygen already allows light to travel throught it with little or no effect to my knowledge. Is the Oxygen almost creating gaps in the silicon and allowing the light to travel through it untouched whilst any of the light that comes into contact with silicon atoms would still be refelected?

I know that might be a ridiculously simple way of putting it but please bare with me.

[another_someone]

That silicon is combined with oxygen does not alter the number of electrons, but the energy of those electrons.

It should also be noted that the fact that glass (we are talking here only about glass - crystalline quartz can be many different colours, depending on its crystal structure and impurities) is normally transparent to visible light, but that does not mean that it is transparent to light outside of the visible spectrum.

It is not about creating physical gaps in the material for light to travel through, but about creating energy gaps.

It may be easier to remember that light is just another electromagnetic wave, just like radio waves.  To receive radio waves, you need to tune the antennae to the right size to pick up the particular radio wave.  If you have too large, or too small, an antenna, you will not pick up the radio wave.  Now think of the electrons as little antennae that are picking up light waves.  When these electrons are in an atom or molecule, the environment they are in has the effect of tuning the electron for a particular frequency of light, but if you put the electron in a different environment, it becomes tuned to a different frequency of light (or, in fact, multiple frequencies, since there are many electrons all with slightly different tuning).  But if none of the electrons are tuned to any frequency in the visible spectrum, then the light in the visible spectrum will go right through.

[londounkm (OP)]

brilliant thank you so much for you patience and your explanation i have a much better understanding now of what it is you are talking about.

With regards to manipulating the energy of the electrons in a given atom, what would be the consequences of this for the atom and the compound.

Take for example the element Gold, would it be possible to change the energy of the electrons to match those of the compound glass so that light could them pass through it or indeed any compound or element?

[another_someone]

It is very difficult to make a metal transparent.

The problem with metals is that some of the electrons in a metal are very loosely bound.  This is what makes the metal a good conductor, but it also means that the electrons can take many energy states, and so will easily be able to reflect lots of parts of the electromagnetic spectrum.

Glass binds its electrons very tightly.  This not only means it is possible to create glass that carefully controls which parts of the electromagnetic spectrum it reflects, but it also makes glass a superb electrical insulator.

Clearly, there are some insulators that will reflect light, but there are no good bulk electrical conductors that are transparent (i.e. you can make thin films of electrical conductors that remain adequately transparent - you may even be able to make things that conduct electricity only in one plane - but conducting electricity freely in the way a metal does would never allow it to be transparent).

[techmind]

... you can make thin films of electrical conductors that remain adequately transparent...

Indium-tin-oxide (ITO) is widely used as a transparent conducting thin-film layer in LCD displays. The ITO is not a stoichiometric compound though (that is to say it doesn't have a well-defined ratio of indium, tin, and oxygen atoms). From what I hear it's a bit of an awkward material to work with (its successful deposition is very dependent on the pre-treatment of the glass, and most other variables you could think of!)



I hope it's not too techie, but my web page http://www.techmind.org/colour/ tries to explain how you get from visible wavelengths to perceived colours.

[lyner]

But if none of the electrons are tuned to any frequency in the visible spectrum, then the light in the visible spectrum will go right through.

The conduction electrons in metals are non-resonant or extremely broad-band resonant. Their possible energy states fill an almost infinite  set of values. That means that they can all be made to move by any incident photon. This means that the surface is a very good reflector. Because of its physical properties, gold can be beaten into a thin enough film (barely more than a few atoms) so most of the incident light doesn't get a chance to interact and just passes through.
The properties of ITO must be due to a much more complicted energy band structure. There must be lots of very low energy state changes possible (a tiny fraction of an electron volt, which would allow DC conduction) and very few opportunities for energy changes of a few eV which would correspond to optical absorption.
It's not just the electrons interacting with em; it's the electrons in their energy relationship with the structure they are a part of.

[graham.d]

Just one point, it is not true that there are no solid elements that are see-through. Carbon in the form of diamond certainly is. I would imagine that there are really quite a few that would be transparent if sliced thin enough. Does anyone know other elements that are transparent to visible light?

[Seany]

I hope this one helps a bit..

http://www.hhmi.org/senses/b110.html