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How Can We See the Blue Light of Day *Only*?

  1. Feb 15, 2013 #1
    Imagine a very big, opaque planar disk blocking the sun's direct rays which is placed horizontally at a high elevation.

    Is it possible that such disk, by blocking the sun's direct rays but allowing the peripheral blue light of the rest of the sky to shine through, would enable us to actually see and bathe in the blue light of day *only*? The disk would have to be very large to exclude direct sunlight impinging too close around the point of observation and drowning out the weaker blue light of the sky (Raleigh scattering light).

    Would we be able to see such blue light exclusively, or would direct sunlight (impinging on the ground at a distance) always be too strong and 'drown out' the weaker blue light? At what elevation would such disk have to be placed and how big would it have to be for the blue light to actually be seen?

    Last edited: Feb 15, 2013
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  3. Feb 15, 2013 #2

    Vanadium 50

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    Like the moon during an eclipse?
  4. Feb 15, 2013 #3

    Yes...except that we want sufficient light to still come through and illuminate the rest of the sky to give us that nice Raleigh scattering blue colour over the portion of sky that is not blotted out by the disk.

    A total solar eclipse would cut off substantially all sunlight; we want enough sunlight around the periphery of the disk to give us a blue sky.

  5. Feb 15, 2013 #4

    D H

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    On a clear day you can see quite well for 20 minutes to half an hour or so after the sun has set (or 20 minutes to half an hour before the sun rises). In some places, you don't legally have to have your headlights on during that half hour or so before sunrise / after sunset -- assuming good weather conditions.

    That pre-sunrise / post-sunset lighting comes from sunlight scattered by the atmosphere. There is no twilight on the Moon.
  6. Feb 15, 2013 #5
    In that case, are you and everything around you illuminated in blue sky light? If you hold up a piece of paper, is it tinged in diffuse blue light? I suspect not...I would imagine you need the full light of day for that effect, hence the big disk to blot out direct sunlight and just let through the blue sky light. Dawn/dusk light may not be strong enough...

    Last edited: Feb 15, 2013
  7. Feb 15, 2013 #6


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    Take a large umbrella and a shovel to the top of the tallest hill in the area. Dig a small depression in the hill deep enough so that the sides block your view of everything but the sky. Now sit inside that depression with a sheet of paper with the umbrella blocking out the Sun and see if the paper looks blue. It probably won't, because there isn't just blue light being scattered, and your color vision isn't solely based on the color of the light, but also of what you expect the color to be based on previous experiences.
  8. Feb 17, 2013 #7


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    Artists certainly think so. The French call it l'heure bleue.
    Btw, Ibn al-Haytham (965-1040) used the duration of twilight to make the first estimate of the thickness of the atmosphere.
  9. Feb 17, 2013 #8
    A large umbrella won't do because we will still have direct sunlight impinging on the ground around us and scattering off to the white sheet of paper, most likely drowning out the blue light effect. That is why I propose a very very large shaded area.

    Also, what if we forget about human colour vision and take a camera with us instead to take a picture of that white sheet of paper? I would still expect a bluish tinge based on this spectrum of blue sky off the Wiki:


    The spectrum is most intense (> 2000) from about 475nm to 600nm. Of that:

    - the range from 475nm to 525nm is substantially blue light;
    - the range from 525nm to 560nm is substantially green light; and
    - the range from 560nm to 600nm is substantially yellow light.

    Green is close to blue in the way of perception and yellow would tend to shift the first 2 colour bands (475nm-560nm) to greenish-blue. Overall effect as per the Wiki: "The color perceived is similar to that obtained by a monochromatic blue of wavelength 474–476 nm mixed with white light, i.e., an unsaturated blue light."


    I believe that white light will not appreciably change a given colour's perception. Therefore, I still believe that we should be able to photograph -if not see- that blue(ish) light on earth...

    Last edited: Feb 17, 2013
  10. Feb 17, 2013 #9

    Thanks for the reference; I have since indeed found several pics of that 'heure bleue', though I suspect many/most have been photoshopped to enhance the blue's appeal...

    I wonder though what the effect would be at mid-day with my above experimental set-up...much much stronger I suspect, and quite a spectacle too :smile: ...

  11. Feb 17, 2013 #10


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    No, that's why you are in a depression on top of a hill. Any ground that could reflect direct sunlight onto you or the paper is shaded by the umbrella.

    Perception is a funny thing. There are many factors that determine the perception of color, so I don't think you can claim that adding white light will not change your perception of an objects color. And I can guarantee you that adding white light will change the way a camera sees an object.
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