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Shadow Colour

  1. Sep 3, 2006 #1
    I noticed this when I was holding a piece of paper in front of my blue led computer light. when a object (finger) is placed in front of the paper wiht the blue led light shining on it the shadow of the object is yellow. why?

    I tried to explain it as yellow is the complementary colour of blue so yellow appears if blue is absorbed but why is it absorbed in the region of the shadow?
  2. jcsd
  3. Sep 3, 2006 #2
    i tried it too...n i saw yellow...or maybe i think i did because i knew it would. it could have been an optical illusion for me. i don't know about ur case though.....hey wait a sec....it is yellow! i think u should try with different colour lights first n then see if u can find the complementary of those colours...how did u think of doing this anyway?:surprised
    i have a question: would the colour of the shadow be black if blue hadn't been absorbed? if it has been absorbed that is...
    Last edited: Sep 3, 2006
  4. Sep 3, 2006 #3
    Yea I know i came across this quite randomly and I though i was seeing things at first. I also want to try with other colours but i dont have any bright enough other coloured leds on me so if someone could try it that would be good. Also I have also tryed this in the dark and the shadow is NOT yellow just black. So ive come to a conclusion that its something to do with the ambient white light or indoors lights.
  5. Sep 3, 2006 #4
    i think that's a fair conclusion. the white light's a combination of colours n when we do this exp in daylight, the complement of the blue light (or any other) get's absorbed and is shown in the shadow. and i think it's shown on the shadow because, there's no where else it can be seen. ie, unless it hits an object.
    i'll try that out for u. maybe i can get different coulours of transparent sheets n cover the front of a torch with them.
  6. Sep 3, 2006 #5
    hey!!! i tried with yellow light n i got a blue shadow!!!!! this is great!!! the yelllow shadow took a little while to be made out but the blue one was very clear at the first sight!
  7. Sep 3, 2006 #6
    Can someone else please also try to explain this? thanks
  8. Sep 3, 2006 #7
    Last edited: Sep 3, 2006
  9. Sep 3, 2006 #8


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    Those are great. Thanks. You might also want to dig around in GD. Evo has posted several links to different illusions.
  10. Sep 3, 2006 #9


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    It's not entirely an illusion.

    Objects everywhere in the room are lit by a combination of ambient light and this extra bluish light.

    But objects hidden from the blue light are still lit, they're just lit by light of a more yellow spectrum. They really are lit by yellower light.
  11. Sep 3, 2006 #10
    It has to do with the way we interpet colors when they are close to other colors. In the example the OP uses he is not blocking ambiant light, he is blocking a source of concentrated blue light. This would mean that the ambiant light is still hitting the shadowed area as it would normally.

    It can be argued that all sight is merely an illusion. I think the examples I posted above show that we can not trust our eyes completely when it comes to color definition. How do you explain those examples, there is no light being blocked at all, they simply arrange colors in such a way that the brain has a hard time telling that the colors are in fact the same when they appear totally different.
  12. Sep 3, 2006 #11


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    Yes. That point has been made. It does not negate my point that there is a real, measurable effect involved. The object in shadow from the blue light really is lit by yellower light.
  13. Sep 3, 2006 #12
    The light is no more yellow that the light hitting the paper outside of the Blue light, yet we wouldn't precieve that paper outside of the blue light to be yellow as the shadow is. They are infact the same color we just interpet them differently.
  14. Sep 3, 2006 #13


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    If you took a colourmeter reading of the light behind the mask blocking the blue light, you would get a colour reading that reads more yellow than a reading where the blue light is visible.

    A camera will capture this effect. It is not merely interpretation.

    See attached, a picture I took downtown of two different-coloured, overlapping shadows.

    Feel free to continue to claim it's a matter of perception. :biggrin:


    Attached Files:

    Last edited: Sep 3, 2006
  15. Sep 4, 2006 #14
    I have no idea what you are trying to show with this image, and I don't understand how it relates to the topic. What the camera captures is irrelivant, we still use out eyes to interpet the image. Have you looked at the pictures in the link I posted? Clearly in the third image one square looks brown, and the other yellowish. They are the same color though as you can tell by using a pice of paper with two holes cut to show only the two blocks in question. Go ahead try it punch two holes in a piece of dark paper that line up with the two centrally located blocks of different color. I would say that's evidence enough that it is a matter of perception.
  16. Sep 4, 2006 #15
    can u people please tell me this? how do u interpret things as optical illusions ? i mean, it could not be an opticall illusion( i'm not relating to this topic) and just something else. take this case for example. it would have never flashed to me that this could be an optical illusion. i thought it was something to do with the spectrum of light.:redface: for all we know, everything we seen in this world could be optical illusions!
    this is so confusing...
  17. Sep 4, 2006 #16
  18. Sep 4, 2006 #17
    i have seen Farsight's link and i can understand why that happens, actually i already knew about that but when i saw this case i couldnt explain with the same logic, can someone tell me if these two cases are the same? cause i am thinking its just an illusion as GOD__AM said
  19. Sep 4, 2006 #18


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    There are several shadows in the picture. They are of different colours. They are different because they are cast by different-coloured light sources. The bluer shadow is cast by red light coming from red reflective building. The redder shadow is cast by the bluer light off a silver-mirrored building, which is reflecting the blue sky.

    This is not matter of perception. It is the same principle as the OP is asking about.

    How this applies to the OP's question is that his yellower finger really IS lit by yellower light.

    You are ramming your pre-conceived conclusion down the OP's throat. The OP wants to know why his finger is yellow. Everyone is telling him it's his perception. I am showing that the phenomenon he asks about can be captured, objectively by a camera. It is not simply about our eyes or perception.

    (Honestly, it's like you didn't even read my post.)

    Yes. And I agree that there IS a perceptual phenomenon. But that is not the ONLY factor here. There is an independent, objective factor.

    Then you come to a hasty conclusion and do a disservice to the OP.
    Last edited: Sep 4, 2006
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