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Is all observable light caused by electrons dropping to a lower discrete energy level while bound in an atom? If so, how can a continuum of frequencies be generated? Is the thermal radiation spectrum continuous, or is it a multitude of "close" discretes?

Qmechanic
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Silliq
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  • Possible duplicates: https://physics.stackexchange.com/q/169209/2451 , https://physics.stackexchange.com/q/73959/2451 and links therein. – Qmechanic Jun 19 '18 at 15:04

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all observable light caused by electrons dropping to a lower discrete energy level

It isn't, all light (visible and not not) is caused by charged particles moving. In atoms they can, as you say, only move between between distinct energy levels and so give distinct frequencies of light.

But a charged particle moving with random thermal motion will change direction, any change in direction (or acceleration = same thing) of a charged particle will give off electromagnetic energy. With a complete range of different motions you get a complete range of frequencies = a thermal spectrum.

edit see What are the various physical mechanisms for energy transfer to the photon during blackbody emission especially Luboš Motl's answer for a more complete description (but possibly beyond what the OP asked)

Martin Beckett
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    Another way to see that steady rise in acceleration and range of frequencies is in black body radiation. The hotter it gets the faster the electrons are accelerated and the higher the emitted photon frequencies can be. – Bill Alsept Jun 19 '18 at 05:01
  • Martin, not sure that this is the complete answer. Planck derived his equation for Black body radiation with the assumption of discrete quanta emission. If something is quantized, in a closed view it can’t be a continuous spectrum. – HolgerFiedler Jun 19 '18 at 05:25
  • This is very intuitive, because I am acquainted with how changing charge distributions cause magnetic fields and an accelerating charge might then create a chirp. I guess this is why free electron lasers are required to generate arbitrary frequencies – Silliq Jun 19 '18 at 05:32
  • Bill, in the solid state, my mental model of heat is that it causes the atoms or molecules to vibrate and move with respect to one another, say stretching and contracting their bonds. Is there a known mechanism for transferring the heat into the electrons? If there is a band gap, then I guess heat would push more electrons into the conduction band – Silliq Jun 19 '18 at 06:05
  • @Siliq All matter is made of atoms and molecules that are constantly moving and vibrating. When you add heat they vibrate and move faster and faster eventually increasing the space between them causing expansion and emitting photons of higher and higher energies as acceleration increases. – Bill Alsept Jun 19 '18 at 06:58
  • @HolgerFiedler changes in momentum of a particle aren't quantized because this would require that the frame of reference (ie space) is discrete – Martin Beckett Jun 19 '18 at 14:59
  • @Martin Good point. In other words, throwing stone of discrete weights with same velocity but under several angles to a detector the measured momentum could be a continuous function. The more interesting point is, is the energy of any electromagnetic quanta aka photon a quantized value. Are the electric and the magnetic fields quantized. If not, how they obey a direction of polarisation. – HolgerFiedler Jun 20 '18 at 20:22