Black Objects

[Ifat Glassman]

Molecules are set up in a structure that allows the molecules to absorb radiation of certain wavelengths, required for electrons to make leaps between different energy levels (Orbitals). Whatever is not absorbed is reflected back to our eyes and perceived as the color of the material. So far so good?

I've been trying to figure out what is the common characteristic for see-through objects, and black objects, but have no answer.

Three known see-through objects are glass, water and diamond: diamond is very organized, glass is amorphous, liquid water are amorphous as well, and the chemical elements are very different as well. So what gives? I can find no common characteristic.

And black objects, as I see it, absorb radiation indiscriminately of the wavelength. Why does that happen?

[DavidV]

According to Wikipedia, opaque objects scatter light more. So the answer may be as simple as the smoothness of the surface.

The example of transparent and translucent objects you mention all have very low compressibility. In fact, black carbon turns clear when compressed into diamond. That suggests that a regular or crystalline structure is conductive to transmitting light.

Edited October 9, 2006 by GreedyCapitalist

[punk]

Molecules are set up in a structure that allows the molecules to absorb radiation of certain wavelengths, required for electrons to make leaps between different energy levels (Orbitals). Whatever is not absorbed is reflected back to our eyes and perceived as the color of the material. So far so good?

I've been trying to figure out what is the common characteristic for see-through objects, and black objects, but have no answer.

Three known see-through objects are glass, water and diamond: diamond is very organized, glass is amorphous, liquid water are amorphous as well, and the chemical elements are very different as well. So what gives? I can find no common characteristic.

And black objects, as I see it, absorb radiation indiscriminately of the wavelength. Why does that happen?

One thing to bear in mind is that substances tend to be opaque at some frequencies and transparent others.

So, to take an example, while water is transparent for the frequencies we see, it is opaque at other ranges.

Actually if you look at the earth's atmosphere, the colors we see fall into a window where it is largely transparent, and the atmosphere is much more opaque outside of that range.

Of course the eye evolved to be sensitive to those frequencies at which the atmosphere is transparent.

Edited October 9, 2006 by punk

[Maarten]

Tentatively I would suggest that if an object is black it generally consists of different molecules that each absorb light at a different frequency, and when you combine them all together they absorb all of the incoming light. I don't know of any molecules that absorb visible light at all frequencies, most substances I know have a very specific absorption spectrum and I've never seen one that was high everywhere.

This is all just a guess, though

[DavidOdden]

Tentatively I would suggest that if an object is black it generally consists of different molecules that each absorb light at a different frequency, and when you combine them all together they absorb all of the incoming light.

So what about non-crystalline carbon?

[Maarten]

Ah, yeah , figures I forget something like that. Okay, scrap that theory

[Ifat Glassman]

Okay ladies and Gentlemen, I have an answer. Thanks to my smart friend who studies physics, I now know.

Quantum levels exist for every matter, for leaps of electrons between different energy levels, energies of rotation, vibration and other stuff of molecules.

Now, suppose there is only one atom. That one atom has a few energy levels for electrons. Now we attach another atom to it: the electrons of the second atom, in the outer-layers (higher energy levels) feel the electrons of the neighboring atom. Uh-oh, they have a competitor for the same energy level. No can share energy levels. So now the level splits into a few close levels, and instead of orbital 3 you now have orbital 3.1 and 3.2 (or something like that, these are just names). Now suppose you add even more atoms: suppose you have a crystal, or a piece of metal, with lots of free electrons. They are all "competing" on the same energy levels that they would have had they been alone in space. The result: a split into many many energy levels that are close to one another. When I say many I mean something like 10^18 many. It seems almost continuous, but it is still discrete.

Okay, now we have the background to understand why some objects seem black (or transparent).

In the case of gasses, they do not have a lot of energy levels, which means that there are only a few wavelength that they would interact with. The result is: they let light go through them without "swallowing" it, and they seem transparent to us (also because they are not very condense).

But in the case of solids: they swallow many wavelengths, because they have many many energy levels. In fact they swallow almost the entire spectrum, and leave just a few wavelengths for our eye to detect, and that is the color of the object that we see (that is not entirely accurate but I won't go into it now).

Materials can swallow a variety of wavelengths for electron-leaps, rotation, vibration and friends. Those objects that swallow wavelengths in the visible spectrum (something between 400-700 nm), are those that seem opaque to us. The more they swallow in this spectrum the more black they appear. Those objects that swallow radiation at different wavelengths seem see-through to our eyes.

Which is exactly what punk said:

Actually if you look at the earth's atmosphere, the colors we see fall into a window where it is largely transparent, and the atmosphere is much more opaque outside of that range.

Of course the eye evolved to be sensitive to those frequencies at which the atmosphere is transparent.

Thank you for what you said about the eye's evolution. I was searching for an answer to that question for some time.

Now, there is another factor, that GreedyCapitalist has mentioned: the surface of the matter.

Polished surfaces cause the light to be reflected in the same direction, which causes them to appear shining. Surfaces that are rough scatter the light in all directions, and it might cause the object to appear darker.

[Olex]

Here is another interesting phenomenon, negative refraction: http://en.wikipedia.org/wiki/Metamaterial#...efractive_index