How to determine the refracting index of a medimum?

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Discussion Overview

The discussion focuses on methods to determine the refractive index of a thin piece of plexiglass, exploring both experimental approaches and the reliability of existing data. Participants consider the implications of measurement accuracy and material properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests using the formula refractive index = sin i/sin r, where i is the incident angle and r is the refracted angle, but notes that measuring these angles accurately can be challenging with a thicker sample.
  • Another participant reports obtaining a refractive index of 1.59 from multiple measurements, which they feel is inconsistent with typical values found online (1.49-1.51), expressing uncertainty about the accuracy of their results.
  • A suggestion is made to use glycerol to match the refractive index of the plexiglass, allowing for a visual demonstration of refraction and providing a comparative measurement at different temperatures.
  • Participants emphasize the importance of considering measurement errors and uncertainties in the reported values, questioning how precise the measurements need to be.
  • Discussion includes the variability of refractive indices in plastics due to factors like fabrication methods, temperature, and birefringence, suggesting that the source of the refractive index data should be scrutinized for reliability.
  • One participant agrees that a measured refractive index of 1.59 is likely accurate, but humorously questions the characterization of 2 cm thick plexiglass as "thin."

Areas of Agreement / Disagreement

Participants express differing views on the accuracy of the refractive index measurements and the methods used to obtain them. There is no consensus on a definitive method or value, and uncertainty remains regarding the reliability of online data sources.

Contextual Notes

Participants note that the refractive index of plastics can vary significantly based on multiple factors, including material properties and environmental conditions, which may affect the measurements and reported values.

callejon
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I need to determine the refracting index of a thin piece of plexiglas. Is there a way to find the index exactly?
 
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Send a thin beam of light into it at an angle so that it refracts. Measure the incident angle i, the refracted angle r, and the refractive index = sin i/sin r. However, if it is very thin, this might be hard to do.

Otherwise, look it up in a databook or the 'net. How accurate a value do you need?
 
yes, I tried to do like that. But the piece is 2 cm thick. I did it 3 times, and I found almost in all 3 tries n=1.59. But this value seems to be very high for the refraction index of a plexiglas. On the web it's given between 1.49-1.51. So I can't be sure. Isn't there a more accurate way of doing that? It would be ok when the error is 0.01.
 
OK, fill a beaker with glycerol (propane-1,2,3-triol) and at 20 degrees this has a refractive index almost identical to Pyrex - the net result is that the pyrex seems to disappear. Try it with your plexiglass, and try different temperatures. If your glass disappears (we do this with Pyrex tubes - it really is a good demo!), record the temp and look up Glycerol's refractive index at that temp.

Otherwise, redo your experiment many times, reduce the errors as far as is possible, and then trust your data!
 
You may also want to consider your measurement errors. What is the uncertainty in that last digit? Or even the second digit.
 
Also remember that you have to take into account any additional rays, i.e. Airy summation of multiple waves.
 
Note that plexiglass is a plastic. Plastics have refractive indices that vary greatly, even across the same type of material. Plastics are also highly birefringent. Both of these properties are due to the molecular arrangements found in polymer materials.

Refractive index of plastics generally depend on fabrication methods, annealing processes, stress, etc in addition to the type of plastic. Note that the refractive index of plastics varies greatly with temperature as well.

I would investigate your source again, make sure it doesn't refer to a specific type of plexiglass that has been prepared in a specific way. Also check its reliability, I have seen quoted figures on the web that are very questionable.

Adrian Baker said:
Otherwise, redo your experiment many times, reduce the errors as far as is possible, and then trust your data!

Agreed, if you measure a refractive index of 1.59, then the refractive index is most likely around 1.59.

Claude.

P.S. Two cm thick plexiglass is thin? :smile:
 

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