[MaxPower2k3]

I don't know if anyone here will know this, but i'm having a discussion with 2 different people about this and none of us really know what we're talking about. There's some smart people on here (i think...) so maybe one of you will be able to help us:


Concerning only solid objects, is the temperature at which something glows a certain color the same for all objects? For example, would a piece of iron and a piece of copper require the same temperature to glow, say, red. This is disregarding any materials that melt/vaporize before they reach this temperature.

Here's my reasoning: Every temperature gives off light. Lower temperatures give off light in the infrared range. At a certain point, that wavelength enters the visible spectrum and we can see it (red). It doesn't matter what material has attained that temperature, it's how hot it is that counts. In a sense, you're seeing the temperature of it and not the object itself glowing. This is how infrared thermometers work. They measure the "color" (in the infrared range) of the light emitted by an object, and can determine, fairly precisely, the temperature of that object. Whether that object is a steak on the grill or the CPU in a computer makes no difference, it's just as accurate either way. this is my totally uneducated reasoning.

My friend says that objects will reach this point at different temperatures, depending on the material. he says that "an increase in kinetic activity doesn't mean that the colors of things will change," but i'm not saying that either. the color doesn't change. An object that reflects green light will always do so (until it changes to some other state, possibly) but when it reaches a certain temperature it will EMIT light. there's a difference there, right?

anyway, i have no idea if anyone here will know the answer, or even understand my poor description, but i figured i'd try.

[f1000]

1. Solids don't emit thermal radiation as a single frequency; they emit thermal radiation as a spectrum. For a blackbody, this spectrum is given by Planck's Law. Although you will perceive a thermal spectrum as being of a single color, it is NOT monochromatic (single frequency).

2. A solid's emissivity varies with material, temperature, and wavelength. A particular material's thermal spectrum will deviate from that of a blackbody in terms of intensity versus wavelength.

Consequently, color alone cannot be used to precisely compare the temperatures of two incandescent materials; the materials' emissivities must also be considered. Your friend is technically correct, but not for the right reasons.

This quote will help to explain the source of your misunderstanding: At sufficiently high temperatures, the radiations of solid and fluid (molten) substances appear to the eye to be almost equal. http://kr.cs.ait.ac.th/~radok/physics/l12.htm

The following link has the best diagrams on emissivity that I could find, http://tes.asu.edu/MARS_SURVEYOR/MGS...missivity.html

[DeathToWindows]

Ok, it's beyond my minimal knowledge in the field, so if you do get a useful answer, let me know.

[Jaey]

I think it's different for each material, but I don't really know much about the topic...

EDIT: (Meaning I could be completely wrong)

[Ozmodiar]

Calling boots to The Lounge. boots to The Lounge, please.

[f1000]

Originally posted by MaxPower2k3:
[B]he says that "an increase in kinetic activity doesn't mean that the colors of things will change," but i'm not saying that either. the color doesn't change. An object that reflects green light will always do so (until it changes to some other state, possibly) but when it reaches a certain temperature it will EMIT light. there's a difference there, right?/B]

An increase in the temperature of an object usually won't cause noticable changes in its reflectivity, but sometimes it does, http://jchemed.chem.wisc.edu/Journal...Jan/abs57.html

In any case, your friend is mixing apples and oranges.

[Logic]

to f1000.

He's correct.

[wataru]

That's more a physics question than chemistry, and you've already gotten an excellent answer so I'll shut up.

[f1000]

Originally posted by MaxPower2k3:
This is how infrared thermometers work. They measure the "color" (in the infrared range) of the light emitted by an object, and can determine, fairly precisely, the temperature of that object.

The IR sensors that I use don't measure a thermal emission's 'color', but its intensity in a narrow band of wavelengths (for the sake of simplicity, think of it as a single wavelength measurement). They must be calibrated for the emissivity of the material they are measuring, and they are only accurate for a narrow range of temperatures (the thermal spectrum must straddle the sensor�s bandwidth). Moreover, if a material being measured doesn�t fill the sensor�s entire field of view, the background temperature may affect the reading.

[boots]

If I understand it right, it's a quantum mechanics question. The Black-Body radiation answer above is correct.

To put it more simply:

1) Electrons have distinct energy levels. Normally, they are in the ground state (or most stable energy level).

2) Adding energy can promote electrons into a higher energy level. The amount of energy needed depends on the difference between the two energy levels in question. The key is that it isn't analog....they don't just get excited in slow progression, they move through steps...kind of like digital vs analog signal. The energy gap between ground state and first excited state is not the same for all solids.

3) When the electrons fall back to the ground state, they give up the same amount of energy that was required to put them in the higher energy state. This energy can be given off as heat, vibration, or (if the energy is right) visible light. The wavelength of the light emitted corresponds the the energy gap. Sodium metal, for example, has an intense yellow because this is one of the primary energy transitions allowed. Lithium, however, is red (I think).

4) If there is enough energy, you can get more than just one higher energy state occupoied. This is where the spectrum comes from. When the electrons go back to lower energy levels, they emit energy corresponding to the difference between the two levels.

5) There is no fifth point. I've already explained everything.

6) Conclusion: Not all solids will glow the same color. And the same solid will not glow with the same color at different temperatures. The higher the temp to which you heat it, the more alternative high energy states you will occupy...so you get additional colors that mix into the glow. Eventually, it will probably just look white to the naked eye.

[mike one]

Originally posted by boots:

To put it more simply:

...

as usual, beautifully put. spot on boots!

glad i didn't have to explain this, guess that is why i'm a lowly organic chemist

[f1000]

Originally posted by boots:
1) Electrons have distinct energy levels.

While this may be true for electrons in a gas molecule, it isn't true for free electrons or electrons in a liquid or a solid. The spectrum of an incandescent solid is continuous.

[boots]

Originally posted by f1000:
While this may be true for electrons in a gas molecule, it isn't true for free electrons or electrons in a liquid or a solid. The spectrum of an incandescent solid is continuous.

It is not true of free electrons (although there are still discrete energy transition such as spin that are still true), but it is most definitely true of electrons in any atomic or molecular configuration. That's basic MO theory. The first energy gap is the HOMO LUMO gap, though other transitions are allowed. That's why we can use things like the sodium D line for a reference standard in refractive index and optical rotation measurements.

Once you give something enough energy to lose electrons, then, all bets are off. Also, in bulk material, you have other absorption/transmission phenomenon that can attenuate the observed spectrum. But the atomic/molecular orbital energy gaps are still there. Some metals have a small band gaps for losing electrons...those that conduct electricity really well, for example, but now you are moving into a different area of physics.

[Weezer]

all this talk is making me remember my orgo class...*shudder*

[f1000]

Originally posted by boots:
It is not true of free electrons (although there are still discrete energy transition such as spin that are still true), but it is most definitely true of electrons in any atomic or molecular configuration. That's basic MO theory. The first energy gap is the HOMO LUMO gap, though other transitions are allowed. That's why we can use things like the sodium D line for a reference standard in refractive index and optical rotation measurements.

No, you are mistaken: the sodium D line comes from vaporized sodium.

According to band theory, the energy levels in a solid, liquid, or highly compressed gas are so broadened and overlapped as to be virtually continuous. Band theory doesn't apply only to metals.

[boots]

Originally posted by f1000:
No, you are mistaken: the sodium D line comes from vaporized sodium.

According to band theory, the energy levels in a solid, liquid, or highly compressed gas are so broadened and overlapped as to be virtually continuous. Band theory doesn't apply only to metals.

I also noted :

Also, in bulk material, you have other absorption/transmission phenomenon that can attenuate the observed spectrum. But the atomic/molecular orbital energy gaps are still there.

We are saying the same thing, just using different language. I'll add, however, that band theory only goes so far. Many bulk materials still maintain discrete transitions that are observable (IR and Raman spectroscopy, for example, rely on this). It is true, however, that these bands are broadened somewhat. Simple applications like sunscreen rely on both discrete absorption and band broadening. Much of this has to do with the structure in the bulk material.

[MaxPower2k3]

thanks everyone. some of this went over my head, but i got the gist of most of it. i think i understand now.

[olePigeon]

<lamen>Differnet solids "glow" at different temperatures. Some don't glow at all and go directly to different state all together.</lamen>

[cheerios]

<layman> lamen = layman </layman>

[f1000]

Originally posted by boots:
We are saying the same thing, just using different language.

No we are not, and the manner in which you discussed line spectra in several of your responses clearly demonstrates your confusion.

With regards to incandescent solids*:

1. Every solid has a conduction band.

2. A conduction band consists of a virtually infinite number of closely spaced energy levels, which are not considered �distinct�.

3. Incandescence is primarily due to the relaxation of thermally excited electrons in the conduction band (I'm fairly sure primarily = entirely).

4. Consequently, incandescent solids always radiate in a continuous optical spectrum.


* Excluding those solids whose dimensions are small enough to be subject to quantum phenomena (nanodots, nanoparticles).



Some other points:


The amount of energy needed depends on the difference between the two energy levels in question. The key is that it isn't analog....they don't just get excited in slow progression, they move through steps...kind of like digital vs analog signal. The energy gap between ground state and first excited state is not the same for all solids.

This is a textbook explanation of photonic emission due to electronic transitions in the atoms of a low-pressure gas (isolated atoms/molecules) and not a solid. The atoms in a metal, for example, have no �first excited states� since their conduction bands overlap with their valence bands (try picking out the D line or any other line from the continuous spectrum of an incandescent block of sodium). Solid ncandescence could very well be described as being due to �analog� electronic transitions.


Not all solids will glow the same color.

At sufficiently high temperatures, all solids will glow the same color (at least to the naked eye). The optical spectra of white-hot incandescent solids are practically indistinguishable (no absorption/emission lines).


In bulk material, you have other absorption/transmission phenomenon that can attenuate the observed spectrum.

Absorption/transmission phenomena do not �attenuate� the optical spectra of incandescent solids. In a dark room, blocks of glass, ruby, carbon, and silver can all be heated to glow the same color.


It is not true of free electrons (although there are still discrete energy transition such as spin that are still true)

In the absence of a field, a free electron in a vacuum does not have quantized spin (I wasn�t referring to �free� electrons in a conductor, which is why I said "free electrons or electrons in a liquid or a solid"). In any case, electron spin �transitions� do not contribute to the optical spectra of incandescent solids.


But the atomic/molecular orbital energy gaps are still there.

Within a conduction band, these gaps are infinitesimal.


Many bulk materials still maintain discrete transitions that are observable (IR and Raman spectroscopy, for example, rely on this

Vibrational and rotational transitions, which IR and Raman spectroscopies analyze, do not contribute to the optical spectra of incandescent solids (at least not in terms of directly contributing photons visible to the naked eye).