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Emission spectrum will change under possible interference?

  1. Jan 28, 2016 #1
    When a molecule makes a transition from high energy state to low energy state, it emits electromagnetic radiation with a certain wavelength, which can be collected as emission spectrum. However, I have a question right here:

    For any real case, there are quite a number of molecules in one experiment. They emit photons under certain excitation wavelength, and these emitted photons are detected. However, is it possible that those photons interference with each other? If so, then what is detected does not reflect the real distribution of wavelengths emitted. Is this possible at all?
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  3. Jan 28, 2016 #2

    Simon Bridge

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    No - photons do not interfere.
    What gets detected is the sum of the emission spectra for all the materials present.

    In principle, two molecules, or groups of molecules, may be glowing - the light from these may exhibit interference ... in which case you get the interference pattern of the emission spectra. It's the same as interference of white light under the same conditions only with fewer colours, and so does represent the "real" distribution of wavelengths. A good example is the spectra of the Sun ... this is composed of the spectra of the elements present.

    It is possible to get a confusing spectra due to the sample being a mixture - this is why we do control experiments.
  4. Jan 29, 2016 #3

    Hi Simon,
    Thank you so much for your help. This is very encouraging. My understanding is that different photons do not interfere with each other, but might interfere with himself. Also, I didn't quite understand your second paragraph, so I am trying to rephrase my question and your answer:

    Consider there are a large group of identical molecules, and when they absorb photons, they will emit photons with a distribution of wavelength. Suppose the emission peak of a molecule is around 500nm. Because so many molecules emit photons with a 500nm wavelength, those 500nm wavelength might interfere with each other and the actual intensity at 500nm will change. Is that possible? Based on what you said, my understanding is that I will always get the interference pattern of the emission spectra and what I observe is still the "real" emission spectra even if there exists interference? Please correct me if I am wrong.

    Thanks a lot!
  5. Jan 30, 2016 #4

    Simon Bridge

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    This is not correct - it is a pop-science garbling of one of the interpretations of quantum mechanics.

    No - you will not expect to see interference of photons with themselves or between individual photons.
    If, however, that 500nm light were to pass through a diffraction grating before reaching the detector...

    If you had a mixture of molecules, some of which emitted at 600nm and the other at 500nm, (so the light from the mixture is yellow!) then that light is passed through a diffraction grating: the central maxima will be yellow, and the others will be doubled with a red and a green line present. This pattern is the sum of the 500nm and 600nm diffraction patterns. You can see a striking example of this by putting white light through a diffraction grating - you get rainbows either side of a white center. Notice also that you never see this effect off white light by itself.

    That help?

    It is also a bit confusing sometimes to talk about photons having a wavelength - that's a hold-over from the wave theory of light.
    Perhaps you are imagining light as being like waves on water?

    It's usually better to describe light in terms of energy and momentum ... that's how you should read the wavelengths when they refer to photons.
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