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Refractive index with a laser?

Posted by Mathieu Butler  
Mathieu Butler October 11, 2018 08:44PM
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?
Doug Daniels October 11, 2018 09:02PM
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).
Frank K. Mazdab October 11, 2018 09:25PM
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...

Edited 1 time(s). Last edit at 10/11/2018 09:27PM by Frank K. Mazdab.
David Von Bargen October 12, 2018 07:42AM
The standard wavelength used to determine RI is the sodium doublet (in the yellow range).
Kevin Hean October 12, 2018 01:28PM
If they do build the better mouse trap, PLEASE let it be digital and not a blurry shadow :-)
Doug Daniels October 12, 2018 09:35PM
David -
That's what I thought. Other wavelengths give slightly different results hence why they are used to determine dispersion.

Edited 1 time(s). Last edit at 10/12/2018 09:36PM by Doug Daniels.
John Attard October 18, 2018 05:50AM
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!
cascaillou December 20, 2018 01:21AM
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.

Edited 2 time(s). Last edit at 12/20/2018 01:23AM by cascaillou.
cascaillou December 20, 2018 01:55AM
oh, and THAT, is the coolest gem refractometer ever made (not anymore unfortunately):

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