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Name: Steve
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Question:
Do all things with thermal energy radiate infrared radiation? If so, does the color of an object affect how this type of electromagnetic radiation is reflected or absorbed? Even though the radiation (reflected or absorbed) is outside the visible range, is it associated with color?



Replies:
You must distinguish the term "color" and the term "wavelength(s)" and/or "frequency(ies)". Color is a complicated response to electromagnetic radiation by the brain, through the eye. The range of wavelengths that can cause a response in the eye is about 400 to about 700 nm. The concept of "color" is much older than the association of "color" with a pattern of wavelengths between 400-700 nm. With instruments we can detect wavelengths (or equivalently, frequencies) of electromagnetic radiation far outside the range of visible light (400 to 700 nm). Various substances, the atmosphere for example, can absorb radiation at shorter wavelengths (ultraviolet ---> x-ray) and longer wavelengths (infrared ---> microwave). Go to:

http://www.giss.nasa.gov/research/briefs/kiang_01/ExtrasolarFig1.gif

Regarding your other question, yes, all objects absorb and emit infrared radiation and ultraviolet radiation as well. These "invisible" frequencies of electromagnetic radiation can have an indirect, but often very easily observed effects. An example is "whitening" additive chemicals to laundry detergents. These components absorb ultraviolet radiation (not visible by the eye), but re-emit blue visible radiation (i.e. light / color). This gives the fabrics a bluish cast, which the brain has been trained to associate with "clean".

Another common experience is the sensation of warmth emitted by a hot body. An electric stove can be adjusted to a temperature so that there is a sensation of "heat", but no perceptible visible light. Here, our skin is behaving like an eye. However, we do not put this in the category of "color" in the usual sense.

Vince Calder


Steve,

If you mean color as our eyes see it, color does not affect absorption or emission of infrared light. If you mean color as some animals see it, it does. The range of radiation our eyes see does not include what we call infrared radiation. Infrared radiation is not special when compared to visible light. It is just at a frequency that does not trigger human eyes. Some animals can see at significantly lower frequencies than can humans. This is part of the infrared range.

If individual atoms, individual molecules, or even patterns of molecules, near the surface of an object can absorb and hold infrared frequencies, the light is easily absorbed. If the surface can absorb the light but cannot keep it, the light will be reflected. If it does neither, most of the light will pass through the surface, perhaps even through the entire material.

If a material absorbs all visible light (dull black surface), it is likely to absorb some of the infrared range as well. If a material reflects all visible light (shiny white), it is likely to reflect some of the infrared range. In both cases, there is no reason to expect a boundary between absorbed frequencies and reflected frequencies to exist right at the "boundary" between visible and infrared light.

Dr. Ken Mellendorf


The central concept here is something called "black body radiation". Essentially, objects emit radiation based on their temperature ('temperature' and 'thermal energy' are very different things, although they are related). Room-temperature objects do emit infrared radiation, but if you heat an object up, it can emit visible light (light bulb filament, heated iron, burning coals), UV light, or other kinds of radiation.

Many factors (chemistry, composition, shape and size, more) influence which types and to what degree an object absorbs radiation. Darker objects absorb more visible light, but there are many other kinds of radiation besides light. Color is a concept people have invented to describe our own sensory experience (visible light by definition is what we see) -- but color is not an inherent property of materials. Visible light is just one part of a large continuum of radiation that can interact with an object. For example, many insects see UV light, so they might perceive an object quite differently than we do. A great example is moth wings. These wings refract UV light, so their wings appear quite different to other moths than they do to people.

Hope this helps,

Burr Zimmerman



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