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Techniques for CollectorsRefractive index with a laser?

11th Oct 2018 21:44 UTCMathieu Butler

I've been wondering why we need an expensive (gem) refractometer to get refractive index, why can't a simple laser pointer be used and the deviation from the norm calculated to get the RI?

I think the laser method can be used for liquids - maybe too tough to get the angle right on solid transparent xls with faces?

11th Oct 2018 22:02 UTCDoug Daniels

Yes, I would think that getting the angle correct using a "simple laser pointer" would be a major problem - even a fraction of a degree off and you get a bogus reading. Also, isn't RI dependent on the color of light used, especially if monochromatic, as a laser pointer would be? (I could be wrong.... never had to determine RI on anything, it's on my bucket list).

11th Oct 2018 22:25 UTCFrank K. Mazdab 🌟 Manager

Yes, RI is dependent on the wavelength of light... the greater the difference between the red and violet ends, the greater the dispersion of a material. But dispersion is tabulated for many materials (or at least should be for materials where it isn't), so use of monochromatic light shouldn't be a problem. Food for thought: an instrument that uses a separate red laser, green laser, and blue-violet laser could be made made into a hand-held "disperometer" (lol). Sounds like someone needs to build this better mousetrap, and then we the mice will come...

12th Oct 2018 08:42 UTCDavid Von Bargen Manager

The standard wavelength used to determine RI is the sodium doublet (in the yellow range).

12th Oct 2018 14:28 UTCKevin Hean

If they do build the better mouse trap, PLEASE let it be digital and not a blurry shadow :-)

12th Oct 2018 22:35 UTCDoug Daniels

David -

That's what I thought. Other wavelengths give slightly different results hence why they are used to determine dispersion.

18th Oct 2018 06:50 UTCJohn Attard Expert

A gem refractometer is inexpensive, much cheaper than most gems!

It is compact and can handle tiny samples. Instead of a sodium lamp one can buy a special yellow light from Gem-A instruments called a "Sodium Line 590nm Refractometer Lamp".

I have one, you can't borrow it, get your own, you will be happy!

20th Dec 2018 01:21 UTCcascaillou

One could get a chinese made refractometer for very cheap (60-100$). However, some of these have calibration issues, so first make sure the seller will accept returns if that would turn out to be the case, and of course make sure you actually have means to check whether the calibration is correct or not (ideally, you would want to compare the reading of a few stones with the reading from another trustable refractometer, but otherwise you might use a flame-fusion synthetic spinel in sky-blue color, which is cheap as dirt and has a rather steady RI, usually 1.720 to 1.730, so if the refractometer is completely off, at least you should notice). Also make sure that the refractometer is either supplied with a monochromatic yellow light source, or with a built-in monochromatic yellow filter (that's usually the case with those cheap units). At last, those cheap refractometer are often supplied with bad quality RI fluid, so be prepared to replace it.

20th Dec 2018 01:55 UTCcascaillou

oh, and THAT, is the coolest gem refractometer ever made (not anymore unfortunately):

https://web.archive.org/web/20110102152221if_/http://nordskip.com/images/refrac1a.jpg

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