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Color through a prism on black and white surfaces.

  1. Sep 24, 2014 #1
    When looking through a triangular prism, I found that a black shape on a white back ground causes the blue end of the spectrum to be on the top of the black shape, and the red/yellow end is directed towards the bottom. The reverse is true for a white shape on a black background. Why is this?
  2. jcsd
  3. Sep 24, 2014 #2
    I'm having a hard time picturing the system you're describing. I'm not sure where the background you're talking about is located or the shapes. Do you think you could rephrase it or draw a picture?
  4. Sep 25, 2014 #3


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    Don't "focus" on the shape, but rather on whatever is white: in your first case, it's the background, in the second case, it's the shape. Without the prism, whatever is white is effectively many different copies with different colors, overlapping exactly. The prism shifts the reddish copies in one direction (the same direction in both cases), the bluish ones in the other direction.
  5. Sep 25, 2014 #4
    To clarify, it's simple a white sheet of paper with a black shape (lets just say a circle) printed on it. The opposite case would just be a black piece of paper with a white shape printed on it.
  6. Sep 25, 2014 #5
    I may be misunderstanding, but I'm not so much concerned with the shape of the image compared to why color arises on the boundary between black and white. I'm aware white light is the combination of colors, but why does this become skewed when black is introduced into the mixture? Part of my issue could also be asking why there needs to be a black image to produce color because simply looking at a white surface through a prism does not reveal color.
  7. Sep 25, 2014 #6


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    Different wavelengths are refracted by different amounts when passing through a boundary between two media. This is called chromatic dispersion. It happens because the speed of light through a medium (not vacuum) generally varies depending on the wavelength.


    Oh, wait a minute...

    Imagine a sheet of white as being a stack of sheets of red, yellow, etc. If you shift them a little bit, by different amounts depending on the color, they'll still overlap over most of their area and produce white. Near the edges, they only partially overlap, and that's what produces the multicolored fringe.
    Last edited: Sep 25, 2014
  8. Sep 25, 2014 #7

    Andy Resnick

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    Sounds like (lateral) chromatic aberration. Glass is dispersive.
  9. Sep 25, 2014 #8
    Thanks! Thinking of the different sheets being shifted really helped me understand it.
  10. Sep 26, 2014 #9


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    The principle of dispersion has been discussed so far but I don't think anyone has 'explained' actually why you can perceive reds on one side and blues on the other when the light is dispersed when, in fact there is an equal amount of spreading for reds and blues at each edge. The same spread of red-through-blue light is actually present on either side of each edge so why don't we see that? It's a matter of our relative perception of a small amount of colour against a black background and against a white background. When the blue parts of the white patch spread over into the black area, they will be more visible against the black background than the reddened (i.e. blue taken away) area left at the edge of the white area. Likewise, when the red at the other side spreads more into the black area, it will be more visible, again because of the contrast than the blue light, diluted by background white. So our eyes 'see' different coloured stripes on each edge.
  11. Jul 15, 2015 #10
    Hmmm... This is an absolutely great topic for discussion, and for a number of even greater reasons. Moreover, still, when it is presented and conducted rightly it becomes one of those extraordinarily rare occasions in any scientific debate when the final theoretical conclusions, as well as the experimental results and observations, complement each other so clearly that in the end no participant or witness to it could--objectively--raise any arguments against them. The only drawback to all that, I guess, is the unavoidable level of 'involvedness' under the current circumstances. Nevertheless, for what is worth I will give it a try (and see where it all leads).

    Before anything else I ought to point out that although one certain fact is crucially important in the matter we're discussing, not one of the thus far participants in this thread (including Liam W.) have made any reference to it. That most important fact is that in order for an observer to see the colours Liam W. saw the spatial orientation of the prism must be with its apex pointing "up". That is the only way for an observer to see the blue end of the spectrum at the top of the prism and the red end towards its bottom, when a so-called subjective prismatic observation (as Liam's was) is conducted.

    Effectively, now, all prismatic observations of light are conducted in 2 ways. First, there is the classic Newtonian way--in which a beam of light is passed through a prism and then projected onto a screen. This is conventionally viewed as an objective observation, and the spectrum it produces is the ROYGBV one. And then there is that so-called subjective type of prismatic observation, which produces a reversed VBGYOR spectrum. Newton was aware of both these spectral distributions, but he only provided some explanation for the so-called objective, ROYGBV one. About the VBGYOR distribution, on the other hand, he never ventured any further than saying “Prismaticall colours appeare in the eye in a contrary order”.... See here[/PLAIN] [Broken] for more, and I will come back (if required).
    Last edited by a moderator: May 7, 2017
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