In case of black-body radiation, radiation gives continuous spectra. Molecular spectra is an example of band spectra. Similarly, there's also the atomic spectra. Why are the spectra of light not same in all cases?
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2$\begingroup$ Light doesn't "emit" anything... $\endgroup$– Lightness Races in OrbitCommented Jan 1, 2016 at 0:16
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3 Answers
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The atomic spectra come from electron transitions. This means that when an atom absorbs a photon its electrons go from one configuration (possibly the ground state) to a configuration with an energy that lies exactly $E_{ph}$ (energy of the photon) higher. The reverse process results in the emission of a photon. The spectrum is a line spectrum, because the electron energy levels are quantized (i.e. only certain discrete energy values are allowed) and the following equation is always fulfilled: $$E_f - E_i = \pm E_{ph}$$
Where the sign depends on whether we are talking about absorption or emission of photons.
Molecules have, with respect to atoms, more degrees of freedom. Imagine a molecule build up from two different atoms (e.g. CO). These molecules, just like single atoms, have electronic transitions. However the molecule also has vibrational and rotational transitions. The vibrational transitions come from the fact that the atoms that are part of a molecule can move with respect of one another (e.g. The C and the O atoms in CO can move towards or away from each other.).
The rotational transitions come from the fact that the molecule can rotate as well. (e.g. the C and O atoms in CO can rotate around the center of mass much like the moon and the earth rotate around theirs.) The energies associated with the vibrational and rotational transitions are also quantised. However the separation between them is much smaller than in the case of the electronic transitions. Because the transitions (and thus the photon energies) lie so close together the individual lines seem to form a band. Therefore molecular spectra are characterized by line (electronic transitions) and by band (vibrational and rotational) emission.
Up till now we have been talking about single atoms or molecules. However the concept of black body radiation is a thermodynamical concept. This means that in order for black body radiation to occur we need to be in the thermodynamical limit: We need many degrees of freedom or put in an easier way, we need a lot of atoms or molecules. The black-body radiation is a property or effect of the whole collection of atoms and molecules in equilibrium with the environment! The radiation originates from the motion of this collection of atoms and molecules. Otherwise known as the bulk.
So even though we were able to more or less link the atomic and the molecular spectra to more or less equivalent mechanisms (transitions between quantized energy levels (this quantization comes from quantum mechanics)). A further "connection" to a black-body spectrum is not possible (because it is a concept of thermodynamics).
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$\begingroup$ "electronic" (from "electron"): Isn't it reserved for other things? $\endgroup$ Commented Dec 31, 2015 at 18:31
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$\begingroup$ I'm not quite sure what else you would use the term for. However it is used in textbooks and syllabuses I read to refer to the process described in my answer. What alternative name did you have in mind? $\endgroup$ Commented Jan 1, 2016 at 8:59
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$\begingroup$ @David VdH Electron, would suffice and be more correct. $\endgroup$– GuillCommented Jan 9, 2016 at 2:46
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$\begingroup$ I looked it up and we are both partly correct. For transitions in atoms the prefered term is electron transition. However in molecules electronic transition is the correct term. $\endgroup$ Commented Jan 9, 2016 at 8:52
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It's where the energy comes from that determines the spectral characteristics. For black-body radiation it is the random collective motion of bulk material constituents. This motion is responsible for an object's temperature which is a statistical measure.
Molecular spectra as well as atomic spectra come from the motions of isolated atoms or molecules that are excited beyond their ground states, but still discrete quantum states. For molecules, it is mostly vibrational or rotational motions of atoms that make up the molecules. For atoms, it is the electrons jumping from one orbital to another.
answered Dec 31, 2015 at 16:19
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$\begingroup$ @Peter Mortensen Thanks for the edit. The post was rushed and I didn't get a chance to proof. $\endgroup$ Commented Jan 1, 2016 at 23:35
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In case of black body radiations , radiations give continuous spectra
No. The radiation is still quantized, it just can't be identified with much ease as in the case of an atom or molecule. Because there are an enormous number of particles on a black body and there are all possible states you can think of. Since the radiation is emitted because a particle moves from one state to another, black body can have all kinds of radiation. We can consider it continuous as the number of particles grew out of the quantum limit so that we can no longer detect the discrete spectra as in a single atom or molecule. In other words a black body is safely above the classical limit so that quantum effects can be neglected with sufficient precision.
