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B What type of spectrum is a rainbow?

  1. Nov 10, 2016 #1
    maybe it is emission spectrum of the sun but, seems continuous... unlike few distinct lines of the hydrogen emission spectrum...few images i just checked about it are similar to absorption spectrum...then maybe difraction could have made emission spectrum seem so or maybe not...
    Well im very confused about it... also include why it is so in the answer
    Anddd.... also why the sun seems white during noon and red during sunset ( im not sure if these two questions are linked to each other but i wanted them answered )
     
  2. jcsd
  3. Nov 10, 2016 #2
  4. Nov 10, 2016 #3
    sooo... what kind of spectrum IS sunlight... emission absorption or continuous?
     
  5. Nov 10, 2016 #4

    DrClaude

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    It is mostly a continuous spectrum (black-body radiation), but there are some frequencies missing due to absorption by atoms in the Sun's atmosphere (this is how helium was discovered).
     
  6. Nov 10, 2016 #5
    Why is sunlight "mostly continuous spectrum" ? if it IS continuous spectrum, why is that so...
    i see that you have said "black body radiation" for that but i cant draw any inference from that, so an explanation would be much more convincing... which is a black body in this case...the sun????
    orrrr maybe... is it so that nuclear reactions give a continuous spectrum...
     
  7. Nov 10, 2016 #6

    Dale

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    Here is a description of blackbody radiation. https://en.m.wikipedia.org/wiki/Black-body_radiation

    It is a continuous spectrum and a good first approximation to a stellar spectrum.
     
  8. Nov 10, 2016 #7
  9. Nov 10, 2016 #8

    DrClaude

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    Because it's a hot ball of plasma.
     
  10. Nov 10, 2016 #9
    so hydrogen in plasma state gives out continuous spectra ? but light from hydrogen plasma is emission spectrum, right? and what about the nuclear reactions of the sun ( maybe... i think they emit light too... ) ?
     
  11. Nov 10, 2016 #10

    Dale

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    Did you read the article? Especially the first paragraph in the section "Spectrum"
     
  12. Nov 10, 2016 #11
    OOps my bad.... so i read it since you specified the part about the sun....my doubts are all clear thanks a lot
     
    Last edited by a moderator: Nov 10, 2016
  13. Nov 10, 2016 #12

    mfb

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    As you mentioned that topic: the nuclear reactions have nothing to do with the sun spectrum. They happen in the core and keep it hot, but no direct radiation from there (apart from neutrinos - irrelevant here) ever reaches the surface.
     
  14. Nov 11, 2016 #13
    Because it is large and optically thick. Any photon emitted from inside the Sun will collide many, many times before it escapes from the Sun. IIRC, it can take millions of years for a photon from the core of the Sun to bounce around and randomly make its way to the edge. In this time, it has collided many times with electrons or protons or groups of particles. Each collision can shift the energy of the photon by a random amount. So a photon coming out can have just about any possible energy. If you plot it on a histogram, it looks like a continuum.
     
  15. Nov 11, 2016 #14
    Yes photons emitted in the core may take millions of years etc. etc. however the implication that any significant fraction of what we see is this deep radiation is wrong. Almost none of that ever does reach the surface. Almost all of the sun light we see is produced in a 100km shell of the sun called the photosphere. There is a thinner atmosphere above this that is cool enough to give the Hydrogen and Helium absorption lines. However at > 5000C the photosphere is a hot plasma and the free charges produce light in something close to a black body spectrum.

    I think this is the answer to the OPs question. Given that the sun is made of hydrogen how does the photosphere produce anything but the hydrogen spectrum? Answer: its too hot for the charge to be bound in Hydrogen atoms and the plasma can make a continuous spectrum. Similarly for any black body, having the ability to interact over the full spectrum is implicit in its being black. If it can't interact with the whole spectrum it isn't black.
     
  16. Nov 11, 2016 #15

    mfb

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    This is a common misconception. 6000 K correspond to just ~1/2 eV, while the electrons are bound at 13.6 eV, and the density is reasonable for recombinations to happen. Most hydrogen (~99.99%) at the solar surface is neutral in the ground state - and transparent to nearly the whole solar spectrum. A small fraction of hydrogen is in ionized states. That still does not lead to emission or absorption lines because both processes are in equilibrium and the radiation mainly comes from the small fraction of ionized hydrogen.
    The absorption lines are produced in layers where the fraction of hydrogen atoms not in the ground state is even smaller.

    More discussion here and also here
     
  17. Nov 11, 2016 #16
    Oops. My bad. I didn't think hard enough about that obviously. Yes, ok. So you are saying enough of the hydrogen is ionized that the light is dominated by the plasma. Got it.
     
  18. Nov 13, 2016 #17
    So why can't we see the missing colors of helium absorption spectrum (emitted by the sun) in a rainbow ?
     
  19. Nov 13, 2016 #18

    Drakkith

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    The absorption lines are too narrow to be seen by the human eye.
     
  20. Nov 13, 2016 #19
    ... but they are there. Search "Fraunhofer lines" or "Fraunhofer spectrum"
     
  21. Nov 13, 2016 #20

    davenn

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    he didn't say they weren't. :wink:
    They just cannot be easily seen without visual aide. For the casual observation, the spectrum looks continuous

    I'm VERY sure Drakkith knows what Fraunhofer lines are :wink:


    Dave
     
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