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We claim that all objects radiate energy by virtue of their temperature and yet we cannot see all objects in the dark. Why not?

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  • $\begingroup$ Your part 2 question leads to the concept of "ideal" blackbody vs. real-world materials. I'd start w/ the Wikipedia entry and select references therein. $\endgroup$ – Carl Witthoft Dec 8 '13 at 15:54
  • $\begingroup$ We use a questions&answer model here. You ask a "question" and you get answers. You don't, then change the question. If you have a new questions you ask a new "question". But don't ask the "how can you have a continuous spectrum?" question because there are already several versions on the site. $\endgroup$ – dmckee Dec 8 '13 at 23:50
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The human eye is only capable of perceiving a very limited range of electromagnetic radiation, with wavelengths ~400-800 nanometer. Objects at low temperatures (room temperature) do not emit an appreciable amount of radiation in this range. The fact that we CAN see objects when it's light is due to reflection. For more info, take a look at this wikipedia page

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Any warm body radiates electromagnetic radiation with a spectrum that depends on the temperature. Above 500 degree centrigrade there is enough radiation in the visible part of the spectrum to be visible but at lower temperatures most of the radiation is at infrared frequencies or lower. Our eyes are not sensitive to infrared radiation so we do not see it.

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  • $\begingroup$ "centigrade" ? How old are you :-) . $\endgroup$ – Carl Witthoft Dec 8 '13 at 15:52
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    $\begingroup$ @CarlWitthoft Check Philip's user page. I was supposedly taunted a few weeks ago by an early twenties guy who said my language was archaic and sounded like something out of the middle of last century. Well DUH - it WAS the middle of last century when I acquired language and some linguistic patterns are pretty much indelible after that! $\endgroup$ – WetSavannaAnimal Dec 10 '13 at 8:52
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    $\begingroup$ Centigrade is still widely used but Celcius is better, Fahrenheit is worse (Google says Centigrade: 1.2 million hits, Celcius: 1.9 Million hits, Fahrenheit: 9.6 million hits) In future I will use Kelvin :-) $\endgroup$ – Philip Gibbs - inactive Dec 11 '13 at 13:37
  • $\begingroup$ I am only in my 40s and I grew up in Canada with "centigrade" and "Celcius" being used interchangeably, but I rarely hear the former now. I always though "centigrade" was a bit odd because it would be entirely reasonable to think of Fahrenheit as a "centigrade" system. The coldest day of the year in (southern!) Canada is about 0F, the hottest is about 100F, seems like a system based on 100 to me! $\endgroup$ – Eric Lippert May 15 '18 at 18:32
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Cold bodies radiate mostly in the infrared zone (invisible to the human eye), but as the temperature increases the body will emmit higher frequencies with more intensity. So room temperature obects will not be seen due to black body radiation.

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As you can see, hot bodies are visible because they emmit visible light mostly.

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  • $\begingroup$ I have thought about this answer, but I have one more doubt: Do objects emit all wavelenghts? I know that that's the case of blackbodies, however, differents elements emits differents wavelenghts, so why can we model any object as a blackbody? Thanks. $\endgroup$ – user35477 Dec 8 '13 at 14:28
  • $\begingroup$ @user35477: AFAIK, all objects can be modeled with the help of idealistic blackbodies. And, Planck's curve is exponential. That means, all objects do emit all wavelengths, but their luminosity (simply, the intensity of light) depends on temperature. The hotter the object is, the more likely you'd be able to see the other wavelengths. In case of human bodies and molten iron, Planck's curve is sharp at IR & visible wavelengths, meaning that such wavelengths have reasonably higher intensities ;-) $\endgroup$ – Waffle's Crazy Peanut Dec 8 '13 at 14:45
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    $\begingroup$ This is a weak answer: all black bodies emit across the full spectrum (as Danu wrote); just that room temp emitters produce next to nothing in the visible range. $\endgroup$ – Carl Witthoft Dec 8 '13 at 15:53
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Human eye can sense a particular range of wavelength of em radiation at room temperature most of the objects do not radiate in that particular range .

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