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Physics/Black Body

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Question
Sir,

A True Black Body is a perfect absorber..can I assume of every frequency..even Microwave through to Gamma Rays? If it is a perfect absorber..why is it also a perfect emitter..can something be an absorber and emitter at the same time ? Kinda counter intuitive?

Thank You.

Answer
First, there is no such thing as a "true" black body.  Yes, the hypothetical object would absorb absolutely every wavelength.  However, every body has some reflectance to light.  And it's not really counterintuitive, as objects heat up they glow.  If you heat something up that is colored black, it will also glow when it gets hot enough.  Black just refers to the absorbance.  Don't get too hung up on the word and your intuition.  Even at room temperature objects emit light, your eyes just can't perceive the weak glow (mostly in the infrared wavelengths).  A blackbody maintains thermal equilibrium by absorbing and emitting radiative energy at the same rate.  

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Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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