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I am looking for a material whose refractive index for the colors green, red and blue to have a significant difference to be visible in simple geometrical optics experiments?! We use a microscope and the apparent depth method to find index of refraction of substances: n= L1/L2, L1: Actual depth L2: Apparent depth

Do you know any substance?

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  • $\begingroup$ It would help if you specified what experiment you were trying to perform. Any prism will illustrate the different refractive index for each wavelength by showing angular separation of colored rays. As mentioned in the answer by John, the Abbe number tells you how dispersive something is. $\endgroup$ – AMCDawes Nov 12 '14 at 19:02
  • $\begingroup$ The indexes are very near to each other, for example: this are some of results for a plexiglass birefringence plate: 1.459, 1.454 and 1.448 in order for violet, yellow and red. I'm looking for substance to have a bigger difference. for example: 1.2, 1.6, 1.9! $\endgroup$ – mini Nov 13 '14 at 20:18
  • $\begingroup$ My main question was about what you hoped to achieve (i.e. why those numbers). This is probably out of the range of any available materials. $\endgroup$ – AMCDawes Nov 14 '14 at 0:19
  • $\begingroup$ @AMCDawes, because of our microscope! I measured those first numbers with a microscope adjustable in the range of micron; Our lab microscopes can only register shifts in hundredth of millimeter that this accuracy is not sufficient to find the depth of the apparent of an object under a red/yellow/... light beam because their differences are in the range of micrometer. $\endgroup$ – mini Nov 14 '14 at 11:08
  • $\begingroup$ That's a different problem, it's an equipment constraint not a materials constraint. Just use a thicker sample, the measured $n$ is a ratio so work backwards from n to find L1 and L2 that you can resolve with your instrument (i.e. more than 0.01 mm apart). You should be able to do apparent depth measurements with a 2 cm sample and resolve with your instrument. If that's hard to find, use a full beaker of liquid. Lots of liquids have resolvable dispersion once they are 5 - 10 cm deep. $\endgroup$ – AMCDawes Nov 14 '14 at 13:55
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The dispersion is normally measured using the Abbe number, so you need to find a material with a low Abbe number. This diagram from Wikipedia shows Abbe numbers for a range of commercial glasses, though the materials tend to get rather esoteric at the low Abbe number end.

Refractive index changes rapidly near optical absorption edges, so any material that is visibly coloured will probably have a very high dispersion. For example I would guess that copper sulphate crystals (if you can get them big enough) would be strongly dispersive.

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  • $\begingroup$ I searched for copper sulphate crystals, and it seems it's not a stable material to be kept in a physics lab for months! $\endgroup$ – mini Nov 13 '14 at 20:22

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