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Refractive index with a laser?

Posted by Mathieu Butler  
Mathieu Butler October 11, 2018 09:44PM
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 10:02PM
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 10:25PM
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 10:27PM by Frank K. Mazdab.
David Von Bargen October 12, 2018 08:42AM
The standard wavelength used to determine RI is the sodium doublet (in the yellow range).
Kevin Hean October 12, 2018 02:28PM
If they do build the better mouse trap, PLEASE let it be digital and not a blurry shadow :-)
Doug Daniels October 12, 2018 10:35PM
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 10:36PM by Doug Daniels.
John Attard October 18, 2018 06:50AM
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!
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