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My understanding was that all objects emit light of continuous spectrum when hot ( black body radiation) but then you see discreet wavelengths in spectra lines and I am confused. I know I am making a mistake somewhere but not able to point it

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  • $\begingroup$ the thing you are missing is that discrete spectra are caused by a different process than the blackbody spectrum. see Anna V's detailed response below. $\endgroup$ Aug 27, 2018 at 17:43

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Black body radiation comes from a large ensemble of particles . The spectrum depends on the quantum mechanical nature of the underlying frame of particles, but it is a statistical effect that can be calculated , and is one of the reasons that quantum mechanics had to be invented. Classical thermodynamics could not explain the spectrum.

black body

The amount of radiation emitted in a given frequency range should be proportional to the number of modes in that range. The best of classical physics suggested that all modes had an equal chance of being produced, and that the number of modes went up proportional to the square of the frequency.

But the predicted continual increase in radiated energy with frequency (dubbed the "ultraviolet catastrophe") did not happen. Nature knew better.

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Spectra are again a quantum mechanical phenomenon, but dependent on the solution of the Schrodinger equation, as can be seen with the fit of the spectra for hydrogen. (the Bohr model was a phenomenological one, before the founding of quantum mechanics as a theory)

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All spectral lines come from transitions between energy levels of individual atoms and molecules bound states. In a hand waving sense, the black body radiation is also due to transitions between energy levels in the lattices of the bodies, but these are dense enough to become a continuum, as the approximation of the black body radiation shows, fitting the black body spectrum.

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You need to multiply the blackbody spectrum with the absorptivity (= emissivity according to Kirchhoff's law) at every wavelength. When the absorptivity of a gas or solid is zero at some wavelength, it won't glow at that wavelength.

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