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Refractive Indexes of Different Colors of Jell-O

  1. Feb 7, 2011 #1
    1. The problem statement, all variables and given/known data
    Why is the index of refraction in lighter colors of Jell-O higher than the index of refraction of darker colors of Jell-O, and why is the speed of light faster in lighter colors of Jell-O?

    2. Relevant equations

    3. The attempt at a solution
    I know the refractive index of the different colors of Jell-O are between 1.30-1.40.

    Note: I'm an 8th grader, I just need help with my Science fair project.
    Last edited: Feb 7, 2011
  2. jcsd
  3. Feb 7, 2011 #2


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    Homework Helper

    Welcome to PF, Imagina7ion!
    Your question is unusual; we usually just measure or look up the refractive index and use it rather than asking why. I am very surprised the color of Jello affects its refractive index.

    You should read the Wikipedia article, which has some information on how the interaction of light with atoms determines the speed of light in a material: http://en.wikipedia.org/wiki/Index_of_refraction
    Note from the article that the index of refraction is c/v where c is the speed of light in a vacuum and v the speed of light in the material. The index is the "slow down factor" for light in a material. Since v = c/n the speed of light in the jello with the highest index will be slowest.

    For your project, you must be extremely careful measuring the index to be sure the effect you found is convincing to the judges. You might let us know how you are measuring the index so we can offer suggestions - it must be awkward to see a beam of light inside the Jello and mark its location. Also difficult to get a flat surface of Jello with an easily measurable direction. Certainly you will need to repeat the measurement at least a dozen times and take an average, perhaps eliminating the highest and lowest measured values.

    Enjoy your project!
  4. Feb 7, 2011 #3
    For my project, I measured the refractive index by:

    1. Positioning Jell-O at an angle(s) (41, 50, 60 degrees)
    2. Shine a laser straight through
    3. Find the angle of refraction (usually around 12-14 degrees)
    4. Substract angle of refraction from angle Jell-O is positioned at (this will be the second angle)
    5. Find the SIN of Angle 1 and divide by the SIN of Angle 2

    To find the speed of light I divided the speed of light by the refractive index of whatever color Jell-O was measured.

    Let me rephrase my question:

    How do different colors of Jell-O affect the speed of light inside the medium? Why is it faster in lighter colors than in darker colors. I am trying to answer this question so I can put it in my conclusion for the poster/final report.
  5. Feb 7, 2011 #4


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    I don't quite understand your setup for measuring the angles. Could you sketch a diagram? The usual way to do it is with a semi-circular "piece" of the material. That way the beam emerges from the material at an angle of zero and does not bend - so you can measure the angle of refraction A2 outside the material.
    There is a way you could make semi-circular pieces of Jello. Visit the school lab and ask for a circular Petrie dish that is divided in half. Fill half of it with Jello and let it set. Then you just ignore the empty half; the light will not bend noticeably as it goes through the thin plastic on the unused half.

    Thinking further about this, I suspect the index of refraction varies with different batches of Jello. More water would very likely reduce the index. More powder would increase it, perhaps even the portions of sugar and gelatin would vary from one box to another. To make a convincing argument that the index varies with color, you would have to mix up Jello batches from a few different boxes, very carefully measuring the weight of the powder and water used. Your school lab should have accurate mass balances you could use.

    Another approach would be to use clear gelatin and food colouring, making all the colors from the same batch. This would deal with the possibility that the Jello company puts more sugar in with one color than another. You could also write to Jello and ask about that.
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