How Can You Tell if Your Glasses Have High-Index Lenses?

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driedupfish
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Recently, I purchased a pair of glasses from a privately owned optic store. I specifically asked for hi-index lens (1.56 or 1.60, I am not sure). The lens I got, however, is kind of thick (around 5 mm thick on the edges).My eye -sight is poor though, -5.00 sphere, -1.25 cylinder on the worst eye. Are there ways to tell if the lens is high-index or just plain regular lens, besides the thickness? Also, there are "rings" on the edge when I look parallel to the surface of the lens.
 
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one reason the lens could be thicker than you expect is if the front surface is flat- some plastic lenses only have one curved surface. This increases the thickness.

I was not able to easily find any technical information about "high index" eyeglass material. I found a few ads claiming n =1.70 or 1.74, but without quantitiatvie information it's hard to separate fact from fantasy.