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Couple of questions on Black body radiations 1. Blackbody emits a continuous spectrum. But blackbody is an idealized body. Do normal objects like a bulb or a table emit a continuous spectrum too.? 2. Does the blackbody radiation curve depend only on temperature.? If I have iron and copper at the same temperature of say 2000 degree Celsius; will they appear of same color.? similarly a table and chair at room temperature will their curve be same too.? I am assuming the different colors we see for the chair and table is due to reflected light. but the curve for emitted radiation will be similar.?

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marked as duplicate by Pieter, Jon Custer, stafusa, Kyle Kanos, ZeroTheHero Aug 29 '18 at 13:53

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Blackbody emits a continuous spectrum. But blackbody is an idealized body. Do normal objects like a bulb or a table emit a continuous spectrum too?

Yes, the thermal radiation spectrum of "normal" objects, also referred to as thermal spectrum, is continuous.

Does the black body radiation curve depend only on temperature?

Yes, that's how the black body radiation is defined.

If I have iron and copper at the same temperature of say 2000 degree Celsius; will they appear of same color? similarly a table and chair at room temperature will their curve be same too?

The relationship between the perceived color due to the thermal radiation and the temperature as well as the shape of the spectral curve, in general, will depend on the emissivity of a material.

Since the emissivity of materials may depend on the wavelength, the spectral curves could be shifted relative to the ideal black body curves and, therefore, the dominant color of such materials due to thermal radiation could be different than the dominant color of an ideal black body or another material at the same temperature.

Besides the shape of spectral curves, the emissivity of real objects is always less than the emissivity of an ideal black body, i.e., less than $1$. In particular, highly reflective, shiny, objects, like a mirror, produce much lower thermal radiation than highly absorbent, dark, objects, like asphalt.

I am assuming the different colors we see for the chair and table is due to reflected light. but the curve for emitted radiation will be similar?

Yes, the material (paint) of a table or a chair could selectively absorb some visible colors and reflect others, the latter will be perceived as the color of those objects.

The thermal radiation energy of these objects at room temperature lies in the infrared part of the spectrum, which we cannot see.

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  • $\begingroup$ I am assuming an object which is reflecting as well as emitting light, the color would depend on out of reflected and emitted which color is dominant.? $\endgroup$ – user31058 Aug 30 '18 at 0:47
  • $\begingroup$ @user31058 If you are talking about thermal radiation at room temp., most of its energy is in IR, so it won't be visible and, in that sense, it won't compete with the reflected visible light and, therefore won't change our perception of color. If IR was visible, than the outcome would depend on the rel. intensity of emitted and reflected light, spectral sensitivity of the rcvr, etc. At very high temp., when thermal rad. energy shifts toward visible spectrum, chances are its color would be dominant, because to compete with it, we would need a very powerful source of the reflected light. $\endgroup$ – V.F. Aug 30 '18 at 1:03
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1: Everything can be modeles as a blackbody with different values for emissivity and absorptivity. A buld have a higher emissivity then a table, therefore it glows and lights up. The spectrum is given by planbcks radiation law $$B_\nu\left(\nu,T\right)=\frac{2h\nu^3}{c^2}\frac{1}{e^{\frac{h\nu}{k_BT}}-1}$$ 2: The curve depends only on temperature (as well as frequency .. to get a spectrum) as we can see above. c is the speed of light, $k_B$ is the bolzmann-constant and $\nu$ is the frequency. If you have iron and copper at the same temperature, their spectrums will overlap, but one will be higher then the other because of the different values of emissivity and abosrptivity. An ideal blackbody have unity for both values.

Your assumption about colour is correct.

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  • $\begingroup$ I am assuming an object which is reflecting as well as emitting light, the color would depend on out of reflected and emitted which color is dominant.? $\endgroup$ – user31058 Aug 30 '18 at 0:46
  • $\begingroup$ @user31058 The dominant color would be the one that is the strongest. If the reflected radiation is stronger than the emitted one this would be the dominant one for example. $\endgroup$ – DakkVader Jul 22 at 8:45
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This answer gives examples of real black body curves, to complete the other answers.

The best fit to the ideal black body curve comes from the cosmic microwave backround radiation, CMB,

cmb

The cosmic microwave background radiation is an emission of uniform, black body thermal energy coming from all parts of the sky. The radiation is isotropic to roughly one part in 100,000: the root mean square variations are only 18 µK, after subtracting out a dipole anisotropy from the Doppler shift of the background radiation. The latter is caused by the peculiar velocity of the Earth relative to the comoving cosmic rest frame as our planet moves at some 371 km/s towards the constellation Leo

The sun is emitting an approximate black body:

sunbb

The yellow is the spectrum outside the atmosphere of the earth, and is approximately fitted by a black body curve, the solid line, of 5778K.

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