[Jewelry making - Article 00017] - Re: [Orchid] Setting - cone shaped hole

[The Doctor]

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>     hmm- my understanding is that gems are cut at angles to _maximize_
>     reflection of light. The principle of optics demand that some
>     light pass through any transparent substance/air interface, even as
>     it requires some to be internally reflected. 

    This is true Betsy, but not in the way you imagine. As I mentioned
    in the last post, faceted gemstones are cut with angles greater than
    their material's Critical Angle, not Refractive Index as has been
    mentioned. What happens in a faceted gem is that when light enters
    through the crown (nobody explains light entering through the
    pavilion because most gems are mounted and worn crown-up), it bends
    and slows down. How much it bends and slows is it's Refractive Index
    (RI). The light travels down to the inside of a (or some) pavilion
    facet(s) and either strikes outside of the material's Critical Angle
    and reflects (and refracts again) off of it, or it strikes inside
    the stone's Critical Angle and exits. If the gem were cut with it's
    pavilion inside the Critical Angle, most of the light would exit
    through the pavilion (Gemologists call this Unplanned Light Leakage)
    and the stone would have no brilliance. 

    To understand critical angle, imagine looking at a horizontal facet
    from the side. Let's say a table facet. Looks like a simple straight
    line, right? Now, imagine a cone protruding 90 degrees perpendicular
    to the center of the facet, apex-up. When light enters inside that
    cone from any direction (obviously from somewhere below), it is
    within the Critical Angle and will leave the gem. 

    Each gem material has it's own Critical Angle and good cutters try
    to polish stones with the best proportions that also take into
    account that gem's CA. If cut properly, you can place any round
    brilliant stone culet-down on printed paper and not be able to read
    the print. Even table-down is difficult with high RI stones. 

    James in SoFl who wonders if my local ice cream parlor has Critical
    Angle cones

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