Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to determine the refracting index of a medimum?

  1. May 21, 2005 #1
    I need to determine the refracting index of a thin piece of plexiglas. Is there a way to find the index exactly?
  2. jcsd
  3. May 21, 2005 #2
    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?
  4. May 21, 2005 #3
    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.
  5. May 21, 2005 #4
    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!!
  6. May 21, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You may also want to consider your measurement errors. What is the uncertainty in that last digit? Or even the second digit.
  7. May 21, 2005 #6

    Dr Transport

    User Avatar
    Science Advisor
    Gold Member

    Also remember that you have to take into account any additional rays, i.e. Airy summation of multiple waves.
  8. May 22, 2005 #7

    Claude Bile

    User Avatar
    Science Advisor

    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.

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


    P.S. Two cm thick plexiglass is thin? :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook