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How do we see things?

  1. Jul 24, 2007 #1
    If you look at a laser beam you are seeing light that is bouncing off particles and scattering all over the place. It goes in all directions and some more then others. But if this happens wouldn't objects farther away get blurry and fade out. That is if all the light is being reflected away on its journey towards the target. If that is true it would explain why it is harder to see through fog.

    Thanks
     
  2. jcsd
  3. Jul 24, 2007 #2

    Danger

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    Not just in fog. Normal distance vision does involve distortion, if only in the loss of resolution caused by the inverse square law of EM propogation. In reality, atmospheric interference contributes greatly to the effect.
     
  4. Jul 24, 2007 #3
    ..... huh?
     
  5. Jul 24, 2007 #4

    xez

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    Yes, the scattering of light from atmospheric aerosols like
    fog, dust, smoke, clouds, et. al. is often a major limiting
    factor in the maximum distance over which things are
    visible.

    "On a clear day you can see forever" is certainly true
    under the best of conditions -- in the day looking
    horizontally you should be able to see things out to
    the horizon defined by the curve of the earth, ~ 35 miles.

    On clear dark nights one can see very faint objects, and can
    see the milky way galaxy's central band as
    such a bright streak that it'd be enough to walk around
    by the light of it on moonless nights.

    Nowdays most often things are indistinct or invisible
    after less than 10 miles or so due to smog / haze / etc.
    near the cities.

    The naked eye is defined to be able to see stars down to
    9th magnitude, but it's rarely the case that that is possible
    in recent decades due to light pollution scattering from
    the atmosphere.
     
  6. Jul 24, 2007 #5

    Danger

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    Maybe distortion was the wrong word there. What I meant was that the photons reaching your eye are not in the same relationship to each other as they were when they left the object. They're farther apart, which drops the resolution.
     
  7. Jul 24, 2007 #6

    russ_watters

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    Sorry Danger, that's not how optics works. The rays of light coming from a point source can be focused to a precise point by a lens, regardless of the distance. The inverse square law doesn't have any effect on that at all - if it did, telescopes wouldn't work. For a telescope, resolution is a function of aperature.

    Maybe you need to see an opthomalagist...? :biggrin:
     
  8. Jul 24, 2007 #7

    berkeman

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    Wouldn't that be optometrist in this case? :wink:
     
  9. Jul 24, 2007 #8
    I like 'anti-distortionologist'
     
  10. Jul 24, 2007 #9

    Danger

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    I don't dispute your expertise in this, but you're going to have to explain something to me. If the photons don't spread out, why do you need a wider lens to see something farther away? Isn't that a matter of collecting the light over a larger area to compensate? :confused:

    By the bye, I do desperately need to get to my optholmologist (who also prescribes and sells lenses).
     
  11. Jul 24, 2007 #10

    rcgldr

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    Lens diameter determines the brightness of the image. A larger lens will collect more light, but this doesn't tranlates into more "resolution".

    Resolution is a function of the perceiving device. If it's a digital camera, then it's the resolution of the camera that determines resolution, regardless of the optics involved. If it's a human eye, then it's the number of receptors in the eye that determines resolution. This is assuming that the lenses used have perfect focus with no distortion.

    Even with no atmosphere, the farther an object is away, the smaller the image (subtended angle). Since the image is smaller, then less receptors receive the image and resolution is effectively lowered. A magnifying lens can compensate for this by increasing the image size.

    You don't need a larger lens, just one with more magnification. The size of the lens determines brightness, not magnification.


    Getting back to the OP, vision within the atmoshpere is limited because of scattering of light, and the effective clarity of atmoshpere over a large distance. In a heavy fog, normal vision may be limited to less than 10 feet (although you'd still see the difference between light and dark).
     
    Last edited: Jul 24, 2007
  12. Jul 24, 2007 #11

    russ_watters

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    Everything was ok except this, Jeff:
    Optics affect the resolution as well due to the the fact that light will diffract when passing through any orifice: http://www.pk3.org/Astro/index.htm?scopes_resolution.htm

    So aperature determines resolution as well as brightness (brightness when divided by focal length).
    Ehh, win some, lose some.
     
  13. Jul 24, 2007 #12

    mgb_phys

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    Although to be fair - only when everything is diffraction limited, which in the real world is only true in high end microscopes (and space telescopes + chip fabrication mask printers).
     
  14. Jul 24, 2007 #13

    Danger

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    It still doesn't make sense to me, so I'm going to slink off somewhere and think about it for a while. :redface:
     
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