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Can we see a photon

  1. Feb 9, 2007 #1
    People say it is possible to see photons.But not one at a time.Is it true

    And also how can the speed of a photon be reduced.Also how can its length be reduced?

    As far as i have learnt from this forum ,a photon duznt have dimensions..

    I read this in an article:

    http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9008999

    Cheers
     
  2. jcsd
  3. Feb 9, 2007 #2
    your seeing them right now ...

    kind of. it depends what you mean by see. for example im looking at a cup of hot chocolate on my desk right now. I can see it. The reason I can see it is because zillions of photons are reflecting off of it into my eyes. your eyes are photon detectors, BUT you can never look out and see a photon. The act of seeing IS photons hitting your eye.

    photons are the particle quantizations of the electromagnetic field and they all travel at the speed of light. The speed of light (in a vaccum) is a constant, but it is different for different materials, like water for example. so a photon traveling through water is moving slower than a photon in space (or air), but it is still moving at the speed of light in whatever medium it is propogating in.

    When you talk about the 'physical extent' of a photon things get a little slippery. light has a dual wave-particle nature. You can talk about the extent of a wave or a wave packet, but not of a photon. very roughly you can think of electromagnetic waves as being huge numbers of photons. And (also roughly) you can think of light as traveling as a wave and being detected as a particle. There are instruments that are sensitive enough to detect a single photon. Actually I think a good human eye is just shy of being able to detect one photon if there is no other light sources around.
     
  4. Feb 9, 2007 #3
    A photon always propagates at c.only the group velocity of light is lesser in a refracting medium.The photons still travel at c!
     
  5. Feb 9, 2007 #4

    jtbell

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    There are many experiments that involve detecting photons one at a time. Try a Google search for "single photon detection".
     
  6. Feb 9, 2007 #5
    anybody have any references on how close the eye is to detecting one photon in the visible spectrum?
     
  7. Feb 10, 2007 #6

    jtbell

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  8. Feb 10, 2007 #7
    Point being: it is not possible to see a photon during its journey to somewhere else. If you see a photon, it's going into your eyes, by definition.
     
  9. Apr 18, 2008 #8
    Light particles?

    When I look into the blue sky on a warm sunny day I see tiny light particles that circle around. Several people I know see them also.
    If not photons, what are they?
     
  10. Apr 18, 2008 #9
    Have you seen an opthalmologist? :-p
     
  11. Apr 18, 2008 #10
    From what I understand the human eye can detect a single photon. See
    http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2006_October_21
    Whether the brain can register it is another story.
    The speed of light has the same speed in all inertial frames of reference. To change the speed you need to place the photon in a gravitational field. Light has a slower speed in mater such as glass and water.

    Pete
     
  12. Apr 18, 2008 #11
    The speed of light locally is always the same, it does not change.
    It is fundamental to the theory of relativity.
     
  13. Apr 19, 2008 #12

    malawi_glenn

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    anantchowdhary:

    How do our eyes work? Well we detect photons, which are emitted or scattered from objects. So we can't "see" a photon, we detect photons - our eyes are photon detectors.

    So in one sense we see photons all the time, and in another sense we never sees a photon since photons don't emit photons and you cant resolve a photon by scattering experiments.

    I would say that it is a matter of defention: "what is mean by seeing something?"


    pmb_phy: The speed of light in vacuum is not reduced by using gravity, you only changes the wave lenght of the light, not the speed. See Gravitational red shift.
     
  14. Apr 19, 2008 #13

    HallsofIvy

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    those are called "floaters" and are imperfections in the aqueous humor inside you eyeball.
     
  15. Apr 19, 2008 #14
    It all depends on what you mean by "see." I think to "see" an object means the eye detects photons that have been reflected off of it. You cannot reflect a photon off of another photon, therefore there's no way to "see" it using that definition.

    I don't think the act of detecting the photon is the same as "seeing" it. If something touches your eye in the dark, you still don't say that you "saw" it. Therefore the photon touching your eye is not "seeing" it. I think there has to be reflection off of it which is impossible with photons.
     
  16. Apr 19, 2008 #15


    I was standing today in the dark toolshed. The sun was shining outside and through the crack at the top of the door there came a sunbeam. From where I stood that beam of light, with the specks of dust floating in it, was the most striking thing in the place. Everything else was almost pitch-black. I was seeing the beam, not seeing things by it.

    Then I moved, so that the beam fell on my eyes. Instantly the whole previous picture vanished. I saw no toolshed, and (above all) nobeam. Instead I saw, framed in the irregular cranny at the top of the door, green leaves moving on the branches of a tree outside and beyond that, 90 odd million miles away, the sun. Looking along the beam, and looking at the beam are very different experiences.


    Meditation in a Toolshed - C. S. Lewis

    :redface: Couldn't help myself.
     
  17. Apr 19, 2008 #16
    nice =]
     
  18. Apr 20, 2008 #17
    hno.harvard.edu/gazette/1999/02.18/light.html
     
  19. Apr 20, 2008 #18
    Yes, more precisely they are usually red cells from the retina's blood vessels or other cells or broken cells coming from retina. It is more probable to see them if you stay some minutes looking down and then you suddenly look up: when those particles move towards the retina for gravity, at a certain point you start looking them.
     
    Last edited: Apr 20, 2008
  20. Oct 7, 2008 #19
    Re: Light particles?

    These are not floaters ... I have seen these too ... just today I got in a discussion with a co-worker about them. I came home and immediately and went online to research, and found your site here.

    Floaters are just that ... they float ... and look like worms or cell-like structures ... and just kind of hang there in my vision (our vision).

    These "tiny light particles" appear as just that! They are FAST and bright and seem to appear and disappear at an amazing rate ... hundreds of them, zooming around. I even thought that perhaps I was seeing a beneficial organism swimming around on or in my eyeball. They are not obstructive to my vision ... I have to make a concious effort to see them.

    That is what this OP is talking about. Are they light photons disintegrating?

    Curious!
     
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