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Diffuse light - clear reflection

  1. Dec 10, 2009 #1
    Think of this one (you can test it) - it's a foggy night and you are approaching a sreet light under which is a puddle of water. Beacuse of the fog you see a diffuse image when looking directly at the light, however, when looking at the reflection, yoe see a clear, sharp image. Why? Can anyone help?

  2. jcsd
  3. Dec 10, 2009 #2
    You may be saturating the receptors in your retina (your pupils are dilated) when you look at the light directly on a dark foggy night, so the reflection is the equivalent to sunglasses.
    Bob S
  4. Dec 10, 2009 #3
    Hmm... this may be right in some cases - I think the effect would happen even with a weak light source, i.e. a candle. How small of a light source would saturate the retina?

    Another comment - this explanation is valid for a nonfoggy night, but in a foggy night I think the reflection should be diffuse even if there is no saturation - the light still has to pass through the fog particles.
  5. Dec 10, 2009 #4
    Correct. Bob S forgot to take polarization of light into consideration.
    When you look directly at the light source, you see the light, and some light scattering off of nearby water particles. When you look at the reflection of the light on the ground, you're looking at only the polarized light from its source--this has the effect of 'cancelling' the light that's being reflected off the water particles, yielding a clear, crisp reflection of the light source.
    Remember that reflections are linearly polarized.
  6. Dec 10, 2009 #5
    Yes, this is what I imagined too. Thanks for the affirmation. However, I have two conceptual problems with this answer also. First, event though I am sure it can be proven, I can't picture in my head how it is that the diffused component gets "cancelled". Fact is that totally linearly polarized light emerges from the reflection only if the angle is right (Brewster angle).

    And second, even the reflected, linearly polarized light has to pass through a certain amount of fog before it reaches the eye, but the image doesn't look diffused. Usually the fog near the ground is considerably less dense than at about streetlight height, so this would account for a part of the reason, but the image still looks too sharp to me for this to be the sole reason.
    Last edited: Dec 10, 2009
  7. Dec 10, 2009 #6
    Perhaps it is a combination of the two forementioned effects that make it so clear - the reflection is less in intensity, so it saturates the retina less? Could this make the reflected image sharper?
  8. Dec 10, 2009 #7
    It is possible that the small-angle scattering of light off of fog is polarized opposite to Brewsters angle reflection off of water, leaving only the direct image of the street light reflecting off of the water. I do know that the polarized light from a really blue sky (Rayleigh scattering) is opposite to the polarized (Brewsters angle) scattering off of water, so the reflected sky looks black.

    Bob S.
  9. Dec 10, 2009 #8
    Yes, that seems like an interesting idea. I wonder if anyone ever calculated something like this, it would be nice to see.
  10. Dec 10, 2009 #9
    Probably not true. The small angle halo around street lights from fog is all around the street light (360 degrees), so there is no average polarization.
    Bob S
  11. Dec 10, 2009 #10
    So, still no clear explanation... bummer, this one is really bothering me.
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