# How to determine the refracting index of a medimum?

• callejon
In summary, to determine the refracting index of a thin piece of plexiglas, you need to do three measurements: incident angle, refracted angle, and refractive index. If the piece of Plexiglas is very thin, this might be difficult to do, but you can look it up in a databook or the 'net. However, if the error is 0.01, then the value you obtain is probably accurate.
callejon
I need to determine the refracting index of a thin piece of plexiglas. Is there a way to find the index exactly?

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.

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?

## 1. What is the refractive index of a medium?

The refractive index of a medium is a measure of how much the speed of light is reduced when passing through that medium compared to its speed in a vacuum. It is denoted by the symbol n.

## 2. How is the refractive index of a medium determined?

The refractive index of a medium can be determined by measuring the angle of refraction when a ray of light passes from one medium to another, and using Snell's law to calculate the refractive index. It can also be measured using specialized instruments called refractometers.

## 3. What factors affect the refractive index of a medium?

The refractive index of a medium is affected by the density and composition of the medium. It also depends on the wavelength of light being used, as different wavelengths will have different refractive indices.

## 4. How does the refractive index of a medium relate to the speed of light?

The refractive index of a medium is inversely proportional to the speed of light in that medium. This means that as the refractive index increases, the speed of light decreases.

## 5. Why is determining the refractive index of a medium important?

Determining the refractive index of a medium is important in many scientific fields, such as optics, material science, and chemistry. It allows us to understand how light behaves when passing through different materials, and can also provide information about the composition and properties of a substance.

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