Glowing materials

Does anyone know the name of the characteristic that causes things to glow and/or change color when heated? I’m curious to compare a number of materials, and I don’t know how to begin looking. Is it as simple as “glow point,” or is there even such a term? Anyway, thanks in advance.

Yxylu

thermochromic perhaps…

Incandescence, I believe.

Both are correct for each part of the question.

Incandescence is the quality that causes a heated object to glow. Stated another way; emission by a heated object that makes it visible.

Thermochromic describes the property of a material to change color when heated.

All materials at any temperature produce light, and at all frequencies, as well. What changes with temperature is the total amount of light produced, and the peak frequency. The body of a typical warm-blooded animal, for instance, is at a temperatur of about 300 Kelvins, or so, and has its peak in the infrared. The amount of light produced in the visual range is negligible, but it’s there. A red-hot stove burner still has the peak in the near infrared, but the peak is close enough to the visible range that there’s still a detectable amount of light in the red range. There’s also some light produced in the blue range, but again, it’s too dim to notice. Heat something until it’s white-hot, and the peak is now in the visible. It’s producing even more red than the red-hot object, but it’s also producing significant ammounts of blue. At blue-hot, the peak is in the ultraviolet or beyond, so there’s more blue light than red (but still more of either than there was at lower temperatures). There’s no sharp cut-off point where you can say “This object glows when it’s hotter than that temperature, but not when it’s colder”.

[hijack]Why anyone posted this in GQ and didn’t email Silver_Fire about it . . . [byejack]

If you are really talking about things that are hot enough to glow (like a light bulb filament or melted metal), then as Chronos exlained, there is no cutoff point. This type of glow is called a blackbody radiation@or thermal radiation, and is dependent largely on temperature and on the property of the material. An ideal blackbody is a black surface that reflects no light.

The properties of the material that need to be considered are absorptivity and emissivity. Absorptivity is how efficiently it absorbs light - an ideal blackbody has absorptivity of 1, and a perfect mirror has 0. Emissivity is how efficiently it emits light - ideal blackbodody is 1, and a perfect mirror (which doesn’t emit any light) is 0. In fact, the emission efficiency and absorption efficiency is always the same - otherwise, if something absorbs more efficiently than it emits, it will get hotter and hotter. So why the two terms? It turns out that these properties are dependent on wavelength (color). Which should make sense - a green object has low absorptivity in the green light (reflects all green light and absorbs none), and high absorptivity in red and blue (doesn’t reflect them at all). Now, in everyday situations, things are at room temperature so they emit infrared. Things are usually bathed in visible light, so we’re mostly concerned about absorption of visible light. So references list the absorptivity of visible light (usually just called absorptivity) and emissivity of infrared (usually just called emissivity).

A) :rolleyes: B) There is no longer an underscore in my name.