Are all wavelengths of light possible?

  • #1

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I just had the thought that atoms emit light at quantized levels but that would seem to imply that only certain energy levels could possibly exist instead of a complete spectrum. But, if light is traveling down or away from a gravitational field the frequency gets shifted. Would this make it such that there can be a continuous spectrum of light but only quantized values in any gives location in the field? Could you use the spectrum of say hydrogen in the middle of nowhere and look for shifts in the emission spectrum to detect something like dark matter?
 

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  • #2
Orodruin
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I just had the thought that atoms emit light at quantized levels but that would seem to imply that only certain energy levels could possibly exist instead of a complete spectrum.
Emission from excited atoms is not the only way of producing light.
 
  • #3
jbriggs444
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Emission from excited atoms is not the only way of producing light.
In addition, not all excited atoms are at rest relative to the eventual observer when they emit light.
 
  • #4
sophiecentaur
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that would seem to imply that only certain energy levels could possibly exist instead of a complete spectrum
The production of a single frequency only happens when the atoms of your gas are very far apart and with no electric interaction. The Pauli Exclusion principle means that no two atoms can have exactly the same energy level and the interaction between adjacent atoms 'splits' what would be a single level into a number of levels. When enough atoms are close together (under pressure or in a solid or liquid), the result is a continuum of levels so there will be a possibility of a continuous band of frequencies emitted.
 
  • #5
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First, black body radiation--the light emitted from a hot enough object is continuous. If you look at the spectrum of the sun, you see mostly black body radiation with a lot of dark bands that are absorption features due to the cool gas between you and the photosphere. There are also a few bright bands that are emission features.

Going in a different direction, inside a container, the possible frequencies are limited by the dimensions of the container. If you and a friend were inside a rectangular metal box with say dimensions of ten by ten by 50 meters, you could not communicate using long wavelength radio waves. (There are resonant effects, such that you might be able to use, for example, 200 meter waves. Your antennas, though, would be the same length as the box.) Lasers and masers use these resonant effects to generate a very narrow bandwidth source.
 
  • #6
sophiecentaur
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First, black body radiation--the light emitted from a hot enough object is continuous.
This is true, of course but for the visible frequencies mechanism of emission is still due to electronic transitions (energy states of electrons in the bulk material rather than a simple electron / nucleus).
 
  • #7
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This is true, of course but for the visible frequencies mechanism of emission is still due to electronic transitions (energy states of electrons in the bulk material rather than a simple electron / nucleus).
(Maybe this will lead to a post to the "Today I Learned" thread for me.) I had thought that the thermal motion of ionized atoms in a star contribute to its black body radiation. Is that not the case?
 
  • #8
sophiecentaur
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(Maybe this will lead to a post to the "Today I Learned" thread for me.) I had thought that the thermal motion of ionized atoms in a star contribute to its black body radiation. Is that not the case?
Well. I never thought in terms of a hot body just consisting of a load of photons that leak out and in from outside . (Lol). Any EM quantum that enters or leaves must (surely?) involve an appropriate interaction involving charges.
I guess it’s not obvious that the spectrum of a hot lump of ‘something’ is actually what was calculated for a hot cavity. I never really considered where it ties in. Hopefully we’ll get a comment from someone who can help me / us.
Actually there are deviations from that spectrum for many hot objects.
 
  • #9
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Another possible source of continuous spectrum not mentioned yet, is recombination of ions and free electrons. The energy of a free electron can have any arbitrary value, therefore energy of the emitted photon (energy of the free electron plus binding energy released during the recombination) is also arbitrary. Thus frequency of the emitted photons in the ionized gas (as in stellar conditions) can continually change as the kinetic energy of captured electrons can vary continually.
 

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