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The lowest visible frquency

  1. Oct 17, 2013 #1


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    what is the lowest visible frequency? can anyone see light with f less than 4*1014 Hz?
  2. jcsd
  3. Oct 17, 2013 #2

    Simon Bridge

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    This is a very commonly asked question - did you try canvassing the answers?

    The longest wavelength most people have a chance of seeing is 780nm ... with differences between individuals.
    Which is around 3.85x1014Hz or 385THz

    That wavelength is incredibly difficult to see - try it.
    Frequencies "less than" that get easier to see ... but I think you meant to ask about higher frequencies :)

    AFAIK: nobody is tracking World records for the longest wavelength of light seen.
    What do you want to know for?
  4. Oct 17, 2013 #3


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    Thanks, just out of interest, actually the bolded part is what interested me most. I must learn to "canvass", I regret I did not know that :redface: , and got a yellow card!
    Thanks again
  5. Oct 17, 2013 #4

    Simon Bridge

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    Googling your question is a good tool, and good training for when you have to do literature reviews.
    Read lots of replies to get a feel for the range of answers to expect - and use this information to refine your question for the next round of searching. That way, by the time you ask a question here, it will be a very well thought out and researched question. i.e. you could have asked about the variation in long wavelength sensitivity between individuals right off the bat.

    To get the variation between individuals, you want to find some actual data ... that would be "spectral sensitivity of the human eye". This is a field with lots of work being done for different reasons but hunting in, say, Google Scholar, will give you an idea of how big-a question you asked :)

    The cut-off wavelengths will probably be quite sharp since they depend on the chemistry of the cones in the retina. 780nm would be "near infra-red" BTW. Longer than that and the light starts to get detected by your body as heat.
  6. Oct 18, 2013 #5


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    Is there really a chance I could do that?, I'd love to
    I am only a student and probably do not belong here.
    I tried googling but it is dangerous for me as I cannot distinguish good sites from bad :
    I found a site on QM that explained things very clearly, I asked in a (probably my first) thread if it was reliable (how could I know?) : the thread was deleted and I was reprimanded as the answer, apparently, was "no"

    In this case I tried at a good site a mentor recommended:
    and it gives ( like wiki and other sites) a figure of 700 nm, which I suspected to be a rounded figure. You confirmed that, but I could not possibly expect such a different figure of 790 , the one you gave me
    And then, I was curious to know if that can vary a lot with individuals.

    This is only in order to justify my behaviour. Please accept my apologies for wasting your time:smile:
  7. Oct 18, 2013 #6


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    bobie, You are NOT wasting member's time here. You've asked quite reasonable questions. You've already advanced your research skills by taking the good advice given by Simon Bridge above.

    In addition to searching the literature you may consider experimenting.
    Can you make a rainbow at home?
    Do you have a prism?
    Can you place it in the sun and display the rainbow on a sheet of white paper?
    How do scientists measure the wavelengths (or frequencies) of visible light?
    Can you imagine an experiment where you invite your friends to look at various "colors" and record the results?
    Can you apply the scientific method?

    That Hyperphysics link is excellent. It will show you accurate and correct physics. Trust that site!

    Keep us informed on your progress, and feel free to ask anytime if you have some questions or doubts. Members here at Physics Forums are always ready and willing to assist anyone who is a true "searcher".

    Cheers, Bobbywhy
  8. Oct 18, 2013 #7


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    You can buy 780 nm laser diodes. Be very careful because light you can't see can still blind you.
  9. Oct 18, 2013 #8

    Simon Bridge

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    780nm diodes are not inexpensive ... take care though.
    You can also construct a simple spectrograph from a diffraction grating... you'll get a rainbow off white light. You can mark off the extreme ends of the color band and use some geometry to work out what the wavelengths were.

    The main trouble is that the long-wavelength response of you eye is not great so you need qute a bright light to see it and that can be dangerous.

    Nonsense. This site exists to help students struggling with this sort of material.
    Don't let the terseness of replies get to you - if I thought you didn't belong here I would not have replied at all.

    That is why you look at lots of answers - yu soon learn which sites are reliable.
    You can also get good info off unreliable sites too ... which is a bit unforntunate.

    hyperphysics is an educational site so it gives the standard limits ... more detailed information usually requires more detailed background knowledge.

    Variation between individuals can be dramatic - with some people unable to see any red light at all.
    But do not expect a great deal of penetration into the infra-red ... the ability depends on chemistry and hard physical laws. For someone to see 790nm, they'd probably have to have an extra chemical in their eyes no-one else has.

    detailed rundown on the human visual system...
    ... scroll down to "absorption spectra of human visual pigments"

    ... discusses evolutionary constraints on what the eye is susceptable to

    But what you want is something that covers "photometric properties"

    .... what you are looking for is
    "spectral responce of the human eye" or "near-infrared response of the human eye".
    https://www.physicsforums.com/newreply.php?do=newreply&p=4542447 [Broken]

    If you are looking into specific claims of being able to see in the infra-red there may be more direct approaches to checking them out.
    Last edited by a moderator: May 6, 2017
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