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Does the color of the refractive medium affect its index of refraction? I want to determine the refractive index of different colors of jello. By keeping as many controlled variables as possible (brand, concentration, volume) and by using a monochromatic light (like a Green or red laser) to measure the angles of incidence and refraction, am I expected to get different results for refractive index, depending only on the color of the medium?

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  • $\begingroup$ The dye used to color the gelatin may well have some (likely small) effect on either the density of the gelatin or more directly on the index of refraction. I'd bet more on the first than the second, but have fun! $\endgroup$
    – Jon Custer
    Jun 1, 2020 at 16:11

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Rule One: Have Fun. Now, on to the optics.

The refractive index is a bulk property of the material which means you cannot affect it with tiny amounts of things like food colorants. You can affect it with bulk constituents, like by adding sugar to water; in fact there is a handy little gizmo called a refractometer which is used by winemakers to tell how much sugar there is in a sample of grape juice, by measuring its refractive index against a scale that you can read by looking into an eyepiece on the end of the thingie. It would be a good idea to get you a pocket refractometer, with which to take measurements and impress your friends at parties.

Anyway, the bulk constituents of the jell-o that will determine its refractive index are water, sugar, and the gelling compound (which is probably pectin).

There will be wavelengths of light which get absorbed by the colorants in the jell-o at the same time they are being refracted by the water/sugar/pectin mix. To make the jell-o look reddish, green and blue have to be absorbed, and this means that the spectrum produced by a prism made of colored jell-o will contain noticeable gaps.

Which brings us to another way to have big fun with colored jell-o: Note that to tint the jell-o different shades of color, you add a mixture of colorants that absorb different wavelengths of light, and then you adjust their proportions until you get the right shade. If your jell-o contains a mix of colorants, then there will be several gaps in the spectrum, and the overall "dimness" of the gap is proportional to the amount of that particular colorant in the jell-o.

And guess what! There is another gizmo called a spectrometer which, like the refractometer, has an eyepiece through which you look to see the whole spectrum of light entering the thingie. These are easy to build (DIY tutorials & inexpensive kits are available on-line) and some of these can be used with a cell phone app to prepare actual charts of the spectra.

So... Now go have fun!

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