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I Basic question about Scattering and understanding colors

  1. Sep 19, 2016 #1
    So I'm studying scattering and size parameters now, I've come to understand that the sky is blue because the size parameter is such that it's an excellent "scatterer" of blue-violet visible light, and horrible at red-orange, that lets pass, and such the sun is yellow within the Earth.
    I've also come to understand why clouds are white, the parameter is such that it scatters all 7 discrete wavelengths of visible light, and by combining them it forms white.
    But this generated a really basic, naive and almost innocent question in my head, how on Earth do big solid objects color work? My pen is huge, but it's blue, it's not because it's particles are small enough that it scatters blue better right?
    I can't avoid feeling extremely stupid by asking this but i need to know, if color of really small particles is given by it's size, how come we have differently colored huge objects? Following the cloud's logic, shouldn't everything be white?
     
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  3. Sep 19, 2016 #2

    Simon Bridge

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    Solids get their colour from their molecular structure.
    Some visible light hitting the object is absorbed (and re-radiated as head - which you cannot see).
    The rest of the visible light is scattered ... the diffuse colour, what you are used to thinking of as the colour of the object, is the result of thaose wavelengths that get scattered off the surface.

    But you are correct, there is more to colour than the simple model you have.
     
  4. Sep 19, 2016 #3
    No, not unless it's made of glass or something like that which can be solid and scatter light.

    To absorb radiation at a given frequency there has to be a corresponding energy transition for a particle in the material. To emit radiation at a given frequency there also has to be a corresponding transition. This is a result of the photon energy equation ##E=\frac{hc}{λ}##. Quantum physics tells us that there can be a shortage of available states at some energy levels. There can even be complete voids at some energy levels. This is predicted by solutions of Schrodinger's equation inside the material. This calculation can be done for some crystals but in general is not easily accomplished.

    If states are rare at certain energy levels then some transitions will be more common than others. That means a material can be better at absorbing some wavelengths and not so good at others. Likewise the material can be better at emitting some wavelengths when it is heated.
     
  5. Sep 19, 2016 #4

    Andy Resnick

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    Oh dear....
     
  6. Sep 19, 2016 #5
    I googled that one and it appeared that, unless I read false information (which is usual on the internet), I did not invent or reach that conclusion by myself
     
  7. Sep 19, 2016 #6
    Ok, thanks, solids get their color not from a size parameter, but from a "structure" parameter, so the wavelength range they scatter is given by that. :)
     
  8. Sep 19, 2016 #7

    Andy Resnick

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    Can you please provide a link to the site that claims there are 7 visible wavelengths?
     
  9. Sep 19, 2016 #8
    OK yes i get what you meant, i KNOW the wavelengths of visible light are within a range (700 to 400 nm was it?) i just tried to discretize them into the common 7 colors we see, sorry if you thought that was a huge mistake, i thought you meant something else.
     
  10. Sep 19, 2016 #9

    sophiecentaur

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    Apparently, Newton himself saw and identified just six different colours but added a seventh because seven is a sexier number than six (more magically significant, apparently). Indigo / Violet????? Who ever actually ever genuinely saw those two, except on a Dulux Paint shade card or in a darkened lab, after some adaptation time? :biggrin:
     
  11. Sep 20, 2016 #10

    Simon Bridge

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    "indigo", was the added colour - it is the name of a plant that you get a kind of blue die from.
    I always thought of it as a kind of product placement... though it was probably more because there was some sort of indigo fad on at the tie he was naming colours.
    https://en.wikipedia.org/wiki/Indigo

    otoh: magenta is, famously, a colour which does not correspond to a wavelength of light - which means we have to distinguish between the human experience of colour and the properties that give rise to them. Magenta is the eye/mind's response to the absence of green light from an otherwise reasonably white source, ie. it is an artifact of the functioning human visual system. The experience of magenta is what allows us to draw colour wheels when the electromagnetic spectrum does not work like that.

    What I said about visible light also applies to non-visible light ... which is to say: the entire electromagnetic spectrum. However, the situation gets more complicated than just what colour something is. ie. gamma rays do scatter from matter ... but also do other things and can have nuclear interactions more than molecular or atomic ones. Questions about colour are usually more about the human visual system than it is about wavelengths anyway, I included this bit just to provide a glimpse into how big the subject can get.
     
  12. Sep 20, 2016 #11

    sophiecentaur

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    The daft thing is that, when you look at the names for the colours on many CIE charts (Google Images "CIE colour space " and you get dozens of different versions of a coloured-in CIE chart) , violet is not placed on the spectral curve. Fact is, it's such a rare thing to see those far-end colours that it's a 'grey area' (pun intended). The most saturated colours we see regularly are on a TV screen and that has a very limited gamut of displayed colours..
    I was interested to see a lack of the word "magenta" on many of those CIE charts. The world is going to hell in a handcart.
    The idea of M(magenta) being -G(green), C(cyan) being -R(red) and Y(yellow) being -B(blue) is a source of confusion for the newcomer to Colour work.
     
  13. Sep 21, 2016 #12
    It is not always necessary to invoke scattering to explain the color of objects. While in some cases there are pigments that scatter light, in other cases simple reflection, transmission and absorption are all that is needed. For example, a fiber that is dyed blue - white light enters the fiber, bounces around and gets redirected back to the observer. The red wavelengths have been absorbed so the fiber appears blue.
     
  14. Sep 21, 2016 #13
    I should have said the red and green wavelengths have been absorbed.
     
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