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Thermal Energy Radiation
Name: Steve
Status: student
Age: N/A
Location: N/A
Country: N/A
Date: N/A
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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Update: June 2012
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