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A little riddle

  1. Jun 14, 2003 #1
    When you hold a piece of paper up to sunlight it looks white. But when we look at the Sun it looks yellow. Who's first with the answer?
     
  2. jcsd
  3. Jun 14, 2003 #2

    drag

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    Could it be because a piece of paper is made out of
    a whole bunch of stuff so we get the combined white color ?
     
  4. Jun 15, 2003 #3
    the yellow light is being reflected away from your eyes so you don't see it.
     
  5. Jun 15, 2003 #4
    I am not sure, but i guess that :
    White color is an equal 'mix' of all the 7 colors of the rainbow.
    The sun does not emit an equal mix, it emits more yellow than other colors (but eventually it emits all colors).
    So you can say that the sun light emits white+yellow, and your paper seems to reflect yellow away (like what sheldon said), so only white reaches ur eye.
     
  6. Jun 15, 2003 #5

    pmb

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    I don't recall ever looking at the Sun and seeing yellow. The light from the Sun has a spectrum which is very much like that of a black body at a temperature of about 5,800 Kelvin. The peak of the plot of intensity vs. wavelength shows a peak aroung the middle of the visible spectrum and is about constant throughout the visble spectrum with the max at about yellow. But it's not that different. The fact that aa piece of paper looks white is a demonstration of that fact since a white piece of paper reflects all colors. Take the "white" piece of paper and place it under Sunlight - it looks white. Now place it under a incandescant lamp - it looks yellow.

    Pmb
     
  7. Jun 15, 2003 #6

    marcus

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    the color the sun looks probably depends
    on the background you see it against

    standing on moon, sun against a black sky-----looks white

    against a blue sky-----yellow (Y and B are complementary colors)

    coming thru cloud or fog it would be against a neutral gray or white background-----would look white


    the light, separated out from any background effects, would
    analyze out to be pure white

    people's color sense has contrast with background built in
    so complementary colors have something to do with how
    colors look---there could even be an evolutionary reason
    for this, some kind of foodgathering or survival thing---never
    thought about it
     
  8. Jun 15, 2003 #7
    There've been enough guesses on this one

    that I think an answer is in order. I was going to post this earlier but someone put up a thread about why the sky was blue, and that was too much of a hint. But that is why, the sun looks yellow because the blue light is scattered out before it hits our eyes. But when we hold a piece of paper up to the sky, it recives the scattered blue light as well as the yellower sunlight. If you put the same piece of paper in the shade it will have a distinct blue cast. Likewise if you shield it from skylight but let the sunlight hit it, it will look yellowish.
     
  9. Jun 21, 2003 #8
    But isn't lamda max for 5800 K in between yellow and green? And due to scattering and also that our eyes are more sensitive to yellow, that we then perceive the sun as yellow?

    And to answer the original question, you are seeing light shinig through the paper.

    P.S. Tyger we are waiting for you to go on the physics Q&A game
     
  10. Jun 21, 2003 #9
    Don't forget that our eyes and brain compensate for a lot of factors. That is one reason many people have trouble learning photography, they don't make allowance for the camera's stupidity in that respect. So our eyes see sunlight as white, even though it doesn't fit the abstract definition of "white light".
     
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