Hi all, And that leads me to this question, Is there a way one can
work out the critical angle from the refractive index of a given
material?
Cheers,
Hans Meevis
http://www.meevis.com
That was the best description of Critical Angles that I've heard. Thanks. I may finally be able to explain it to my customers now, in layman's terms. Can you explain, to a non-faceter, how Critical Angle is determined? Is there a chart available for various materials? Can this be measured in a cut gemstone?
Douglas,
You’re welcome, I’m glad it can help with your customers. In fact,
I’m glad your customers are even interested in it. Most of mine have
trouble understanding why a 1ct ruby is physically smaller than a
1ct tourmaline, much less the properties of CA or RI. When I try,
their eyes just glaze over.
I couldn’t explain in layman’s terms much at all about how CA is
determined, but basically, it’s a function of Snell’s Law. The
critical angle can be calculated by taking the inverse-sine of the
ratio of the indices of refraction. The formula equating CA to RI is
expressed as:
critical angle = sin-1(1 / n),
where n = RI, and as I’m sure the list’s editing software won’t
display the superscript in this formula, this probably won’t be much
help to you. I guess the simplest way to express it is to say that CA
is the smallest angle of incidence (the angle at which light strikes
the facet in relation to the Normal, which is the line perpendicular
to the facet itself) at which total internal reflection occurs. . I
don’t know of a way to directly measure CA in a gemstone, cut or
rough. As far as I know, materials are measured for RI and those
numbers plugged into the formula to calculate CA.
The good news is that even faceters don’t bother much with
understanding how CA is determined, as there are certainly charts
available for most common materials’ CA. One such chart is on page
23 of Fundamental Faceting, a book available from Graves Company. It
lists around 40 or so popular faceting materials, along with the
material’s Critical Angle and data for round brilliant cuts relating
to Star, Crown Main, Crown Break, Pavilion Break and Pavilion Main
facet angles. They have plenty of other books on faceting that would
likely have more info. Check out www.gravescompany.com contact the
folks there and ask. They’re very friendly and quite knowledgeable
about everything to do with faceting. Usual disclaimer applies. I’m
just a happy customer.
You can also check with the US Faceters Guild
www.usfacetersguild.org too, They may have an online chart. I’m
sure they probably have all the info you want on CA, too. In fact, I
just went to their site and browsed an online version of their
downloadable freeware Faceter’s Companion CD. There’s an online
chart there you can peruse that has RI, CA and many other properties
for commonly faceted gems. Link:
http://rockhounds.com/oplc/cd_online/gemstone_properties/index.html
Hope this helps you Douglas. Just remember, when the customer’s eyes
roll upward and begin to glaze over, switch the conversation to how
pretty it looks
James in SoFl where the CA of rainwater isn’t quite as important as
the flooding it’s been causing around here.
Hi all, And that leads me to this question, Is there a way one can work out the critical angle from the refractive index of a given material?
Hi Hans…
Here’s a formula I found in a wonderful old book put out by Lapidary
Journal… “Practical Gem Knowldge For the Amateur” by Charles J.
Parsons…my edition is 1969… I believe the book is out of print,
and by today’s standards is a homely book…but if you can find one,
snap it up…it’s full of all kinds of good stuff…
At any rate…to find the critical angle of a material, given you
know the R.I. :
1 divided by the RI = the sine of the critical angle…or
1/RI = sine CA…
Diamond RI = 2.417
1/2.417 = .4137, a look at my handy dandy trig tables (like it
better than calulator for this) show that to be the sine of 24deg, 26
minutes…(.4163 is value in the table closest)…
Better yet, if you have trig tables, they’ll show a value called
cosecant…
If you take the RI, again 2.417 (diamond) and find that value as
cosecant… or as close as you can get…in my tables that’s
2.4176…
You’ll get…angle 24deg, 26 min…
Hope that helps…
Gary W. Bourbonais
AJP (GIA)
Hi Hans,
And that leads me to this question, Is there a way one can work out the critical angle from the refractive index of a given material?
Here’s the formula to calculate the Critical Angle (CA) when the
Refractive Index (RI) of the stone is know.
r = the critical angle
n = the RI
since r = 1/n
Example: RI or tourmaline is 1.63
1 divide by 1.63 =0.6136
0.6136 is the natural function (sine) of 37 deg 51 minutes.
The CA or tourmaline is 37 deg 51’.
The calculation can be done using a calculator with trigonometric
functions or by looking up the sine value in a table of
trigonometric functions.
Dave
And that leads me to this question, Is there a way one can work out the critical angle from the refractive index of a given material?
Sure but it’s a lot easier to look it up in a chart such as:
http://www.usfacetersguild.org/gemstone_properties.shtml
It lists the RI and critical angle of all commonly know gemstones
along with lots of other parameters for identification purposes.
js
Doug, I don't mean to rain on the parade, but I read the same description, and even though I have a vague general understanding of how it works, I was left as befogged as before. I appreciate the effort, but... I totally didn't get the cone business.
Not sure what the cone business is but the critical angle is
determined by the RI of the material. All one needs to know is that
if the critical angle is exceeded, light is not reflected back out
the crown and passes through the stone and is a loss of brilliance.
This is obvious when looking into the table when backlighted. The
stone should look like a black hole. If you see light, it is
defective.
I used to think that all the facets were simply reflecting light
like a chandileer dangle just to make the candles more efficient.
This is not the case with a brilliant cut stone. The light must be
reflected by the pavilion after passing through the table. What this
does is to turn the stone into a prism which spreads the light out
into the colored spectrum and produce “fire”. The higher the RI and
dispersion, the more spreading and the more fire. It is also easier
to produce stones or normal proportions without exceeding the
critical angle if the RI is high… so the RI actually does two
things for a stone… makes it easier to produce brilliance and
produces more fire.
I started out cutting marbles for practice and quickly switched to
CZ when all this became very obvious. A properly cut CZ out
performs most of the diamonds I have seen since I started doing this.
This says less for diamonds than for the quality of what my friends
think are beautiful stones.
js
Hi all, And that leads me to this question, Is there a way one can work out the critical angle from the refractive index of a given material?
Hi Hans,
Yes, see yesterdays’ post. Better yet, visit Bob Keller’s web site
at:
http://www.rockhounds.com/rockshop/gem_designs/refractive_index
Great info, and there is more about gems and gemology throughout the
entire site. The bottom of the page also has some info about
birefringence and doubly refractive stones. If folks thought CA was
confusing, that subject will only double it. And if you want even
more gemology tortuRe:
http://www.yourgemologist.com This page has about as much info on
gemology as you’ll ever find online.
James in SoFl who needs to get back to work instead of trying to
think of something clever to type here.
If you have ever seen a tube set or flush set stone, while it is on the finger it is not receiving any light from the back. That is the simplest way to understand that if a stone is cut properly, it never has, never is, and never will be refracting or reflecting light from the back. I explain to my customers it is the same as a mirror. Light behind the mirror cannot be observed from the front.
This is where it gets so confusing. Textbooks examine one highly
focused, minuscule beam of light as it enters the crown of a
gemstone and follow it through. And yes, a beam of light with those
properties, when directed through the crown of a perfectly (not just
properly) cut gemstone will reflect and refract off of many facets
inside before striking that last crown facet inside it’s Critical
Angle with none of that light leaving through the pavilion. The
reality, however, is different.
A mirror is usually a piece of very flat glass with an extremely
reflective opaque coating behind. Gemstones have no such coating
(well, not the ones we’re talking about here, anyway), and while
they are cut so that their pavilion facets reflect as much light
through the crown as possible, none of them are worn as jewelry in
such a manner as to only allow light passage as the textbooks
illustrate. The visible light travels in both waves and particles,
and from all directions, not just one beam of light from a fiber
optic cable. This means that no matter how perfectly a gem is cut,
some light will always leave/return through the pavilion since some
of it enters at an angle favorable to the phenomenon. Even in a tube
setting, some light can and does leave the pavilion, reflect off the
setting and return through the pavilion and out of the crown for our
eyes to see. That’s why the same stone appears slightly different
when compared in different colored metals. That’s also why gem
graders and appraisers prefer unmounted stones. Most commercial CZ
diamond color grading kits are supplied with one gold color and one
silver color mounting for this very reason, to compare stones
mounted in different colored metals against the masterstones.
One of my previous posts mentioned that gems with pavilion angles
cut less than it’s CA will allow most of the light entering through
the crown to exit the pavilion, known as Unplanned Light Leakage,
which results in pale areas of color in the stone known as
Windowing. Well, some very dark stones are purposely cut this way to
take advantage of CA to lighten it so it will have a more pleasing
look. This is called (of course) Planned Light Leakage. The point
I’m trying to make with this is, the bulk of the gemstones we see
are seldom cut to perfect proportions and while studying gemstones
many things apart from what is illustrated in the textbooks for a
perfect situation are what we encounter in reality. And even if a
gem is cut properly, it can, has, is and always will be reflecting
and refracting at least some light from the back.
If I led anyone to believe that every single wave or particle of
light that enters the crown of a gemstone is reflected solely back
through the crown, that was not my intention. Obviously, the visible
light of our everyday world doesn’t behave like a gemological
laboratory’s fiber optics. However, gem setting should require
understanding of the basic principle of Critical Angle if we’re to
explain to our customers that want to know: “Why does this stone in
the tube setting look brighter that the one in the prong setting”?
After all, this thread IS about gem setting 
James in SoFL who is experiencing some Gray Matter Leakage from
thinking about all this stuff.
Is there a way one can work out the critical angle from the refractive index of a given material?
You can derive the critical angle for the interface of two materials
from the refractive index of both.
Sin(CA)= n2/n1
Where n1 is the refractive index for the more dense material (in
this case a gem of some sort) and n2 is the refractive index of the
less dense material (in this case air).The refractive index of air
is approximately one so?
Sin(CA)=1/n1
Take Care
Scott S.
I appreciate the effort, but... I totally didn't get the cone business.
Noel, it’s basically a description of the angle at which light can
exit a gem material (or any material, for that matter. When most
people read about Critical Angle, they usually think
two-dimensionally, as in triangles, etc. With our subject, the angle
we’re talking about is a three-dimensional cone.
To understand, you first have to accept that light can enter a
gemstone. A textbook will treat it as a single beam, not the
scattered thing it is. This is for clarity of explanation and can
really be best understood that way. Anyway, once inside the
three-dimensional gemstone (let’s use a Tourmaline, Critical Angle
38 degrees), the beam of light keeps moving until it either reflects
off of another facet inside the gem, or it exits.
As this beam of light approaches the facet (still from inside), it
does so at some angle. Imagine that you are inside the gemstone
riding on the beam, right up front at the leading edge and coming on
fast! As you near the facet, you see a phantom cone shape extending
precisely perpendicular from the facet. The apex of the cone touches
the facet and the angle of the cone is 38 degrees (You’re inside a
Tourmaline:-) If your angle of approach relative to that cone angle
is greater than 38 degrees, the cone will pass by underneath you,
and you and the beam will reflect off the facet and head for another
one.
Now, you’ve reflected off the facet and are now traveling toward the
next one. Suddenly, another phantom cone shape extends from the
facet ahead of you. This time, though, you’re approaching the facet
at 36.78359 degrees (inside the critical angle). Yes, this time, the
cone envelops you as you travel within it’s shape before exiting the
stone inside it’s Critical Angle.
If this doesn’t help, dear Noel, try this: Get a cone, any cone.
Flatten the apex a little, just for this illustration. Place it,
base down, in front of you. Then place anything flat on the apex. A
tile, perhaps, or a piece of stiff cardboard. Imagine the flat thing
is a facet, and the cone represents the CA. The cone must be
perpendicular to the facet, so don’t forget to use one with a
flattened apex. Now, any light traveling toward the flat thing from
underneath at an angle (or from a direction) outside the cone will
reflect. Anything traveling toward the flat thing inside the cone
will exit.
James in SoFl who would love to ride a beam of light, just once.
This is where it gets so confusing. Textbooks examine one highly focused, minuscule beam of light as it enters the crown of a gemstone and follow it through. And yes, a beam of light with those properties, when directed through the crown of a perfectly (not just properly) cut gemstone will reflect and refract off of many facets inside before striking that last crown facet inside it's Critical Angle with none of that light leaving through the pavilion. The reality, however, is different.
The reality is only different with an incorrectly cut stone. There
is no “perfect” but there is a range of angles that work, i.e.,
anything below the critical angle. It is either totally reflected or
passes through.
What you are refering to is called ray tracing and it is a way to
visualize what light does when it enters the stone. It does not
bounce off of many facets after passing through the table. It
strikes only two and returns thrugh the crown.
A mirror is usually a piece of very flat glass with an extremely reflective opaque coating behind. Gemstones have no such coating
But a prism has no such coating and is totally reflecting, 100%.
and while they are cut so that their pavilion facets reflect as much light through the crown as possible, none of them are worn as jewelry in such a manner as to only allow light passage as the textbooks illustrate.
They can if properly cut. Just load the design into a program such
as GemCad any you can futz till sunrise and not find an entry point
that passes through if the angles are correct.
Even in a tube setting, some light can and does leave the pavilion, reflect off the setting and return through the pavilion and out of the crown for our eyes to see.
That defies the laws of physics and the basic definition of critical
angle.
That's why the same stone appears slightly different when compared in different colored metals. That's also why gem graders and appraisers prefer unmounted stones. Most commercial CZ diamond color grading kits are supplied with one gold color and one silver color mounting for this very reason, to compare stones mounted in different colored metals against the masterstones.
I suspect this has more to do with the contrast of the background OR
to compensate for poorly cut stones with leakage.
There is no excuse for a leaky CZ as the material is so cheap there
is no need to cheat on the angles. The cost of diamond rough motivates
cutters to maximize the yield instead of getting the angles right.
One of my previous posts mentioned that gems with pavilion angles cut less than it's CA will allow most of the light entering through the crown to exit the pavilion, known as Unplanned Light Leakage, which results in pale areas of color in the stone known as Windowing.
That is a defect in a brilliant cut stone.
Well, some very dark stones are purposely cut this way to take advantage of CA to lighten it so it will have a more pleasing look.
Right but it is no longer a brillliant cut. Dark stones absorb so
much light that there is no point worrying about the critical angle
because so little returns anyway. They are usually cut to present
the best color and ignoring brilliance and fire.
js
Hi,
here are definitions of the critical angle etc here at ganoksin:
best
Charles
Charles Lewton-Brain/Brain Press
Box 1624, Ste M, Calgary, Alberta, T2P 2L7, Canada
Tel: 403-263-3955 Fax: 403-283-9053 Email: @Charles_Lewton-Brai1
Ahah! Thank you, James-- now I get it! When I pictured the original
cone, I pictured it upside down, which didn’t help at all. Thanks
for the expansion and the patience… but then… if you’re The
Doctor, you’re supposed to have patience, uh, patients… Anyhoo, now
I find it helpful.
–Noel
Dear Orchid Readers,
I originally responded to this thread for the purpose of trying to
help explain to a person who wonders why bead-set gems are set with
a bur to make a seat, instead of using a straight hole. It evolved
from there into my trying to help explain why Critical Angle is an
important part of gem proportions and worth considering when
bead/tube setting. I never set out to change or defy the laws of
Physics, just trying to help some people understand some stuff. I
know this post is wordy and consumes a lot of bandwidth, but I
sacrificed brevity for clarity (I hope), and I also hope I helped
somebody to better understand at least something. This is a fairly
large post, so those of you who care nothing about Critical Angle
and how light interacts with please feel free to skip
this one.
This is where it gets so confusing. Textbooks examine one highlyfocused, minuscule beam of light as it enters the crown of a
gemstone and follow it through. And yes, a beam of light with those
properties, when directed through the crown of a perfectly (not
just properly) cut gemstone will reflect and refract off of many
facets inside before striking that last crown facet inside it’s
Critical Angle with none of that light leaving through the
pavilion. The reality, however, is different.
The reality is only different with an incorrectly cut stone. There is no "perfect" but there is a range of angles that work, i.e., anything below the critical angle. It is either totally reflected or passes through.
As I said, a tightly focused beam of light will do just that.
Problem is, we don’t typically wear jewelry in that situation. I’m
talking about real light that travels in both particles and waves,
not what the textbooks describe for clarity of explanation. And to
reflect, the beam has to hit outside the CA, not below as you
stated. I don’t care how well a stone is cut, the nature of the
light under which we view gemstones on a daily basis does not arrive
at the gem in the way it is most easily explained in the books,
therefore, it doesn’t leave in that precise way. Best example:
Charles Lewton-Brain recently posted this URL in this thread:
and in the second paragraph under his discussion of light at the
start mentions that while the Particle Theory of light has
superceded the Wave Theory, the Wave Theory is still best for
describing how light interacts with It should be plain
that not only is the textbook example an oversimplified explanation,
but also that the reality of what happens when both aspects of light
are considered is actually different from theory. If you’re
understanding this with only the Wave Theory, you’re missing the
rest of the picture.
What you are refering to is called ray tracing and it is a way to visualize what light does when it enters the stone. It does not bounce off of many facets after passing through the table. It strikes only two and returns thrugh the crown.
No, I’m not referring to raytracing. Raytracing is actually a method
for producing views of a virtual 3-dimensional scene on a computer.
It tries to mimic actual physical effects associated with the
propagation of light, but it falls somewhat short of reality. GemCad
is one such application and is a boon to faceters. So is LightWave,
which is often used to produce the CGI effects you see in films
today. I’ve no experience with GemCad, but I did use LightWave in a
previous career. Nope, what I’m talking about is an actual
laboratory experiment I observed where a beam of light was focused
on the crown of a diamond which did, indeed, strike two pavilion
facets before exiting back through the crown. However, when the
angle of the beam relative to the table facet was changed from
perpendicular, the results varied much more, including multiple
internal reflections before exiting either crown AND pavilion.
Visible light, whether from our sun or artificial lighting is seldom
focused tightly on our gems from directly over the table facet and
is most certainly not raytraced. It comes from all directions, all
axes (X, Y and Z) and is far more complex than a personal computer
can raytrace. Raytracing is a very useful tool for computer
modeling facet designs, but it doesn’t take into account abberations
caused by many contributing factors like inclusions and color
centers, or the small but optically significant variations that
occur in real-life gem cutting.
A mirror is usually a piece of very flat glass with an extremelyreflective opaque coating behind. Gemstones have no such coating
But a prism has no such coating and is totally reflecting, 100%.
Prisms are cool. A prism is a polyhedron with two parallel faces
called bases. The other faces are always parallelograms. We’re
talking about which to my knowledge are seldom, if ever
cut that way. Mirrors are cool too, but not as cool as prisms. The
original poster said that a gem’s facet is the same as a mirror’s,
but we all know that’s not true. It may be a simple way of not
confusing the customer, but all you’re trying to do here is get
off-topic and catch me out. Sure, a gemstone refracts light in the
same way as a prism does, and it also reflects off the pavilion (or
crown) facet if, and only if, it strikes that facet outside the
material’s Critical Angle. It’s the same with a prism: Light enters,
strikes a surface outside it’s CA, reflects (and refracts) to
another surface where it strikes inside it’s CA and exits. The thing
is, here in the real world, light strikes a gem from all angles, not
like a raytraced computer image. Prisms are made with dimensions and
angles favorable to 100% reflection. Gemstones are designed with
dimensions favorable for the same, but their shape and dimension
vary widely from prisms. With a mirror, the back is opaque (faceted
gems usually, but not always, aren’t) and highly reflective, so no
light can pass through the back no matter what angle it strikes the
surface. In a gemstone, it has a chance to exit the back, in a
mirror, it doesn’t. In a prism, it has a 100% chance to exit, which
proves my point, so thank you for the example. But you’re also
comparing a mirror, a gemstone and a prism without addressing
important relativities such as Crystal Plane d-Spacings, Interplanar
Angles and Crystal Lattices as applied to those specific materials
and it clouds the real issue, which is how visible light interacts
with a faceted gemstone.
and while they are cut so that their pavilion facets reflect as
much light through the crown as possible, none of them are worn as
jewelry in such a manner as to only allow light passage as the
textbooks illustrate.
They can if properly cut. Just load the design into a program such as GemCad any you can futz till sunrise and not find an entry point that passes through if the angles are correct.
Again, I said “as the textbooks illustrate.”, which is usually with
the Wave Theory. Not as GemCad illustrates. The light under which we
view gems in reality are not raytraced in a computer. You can futz
till sundown (because you’ll need the light) and a gemstone, however
cut, will not behave in reality (as opposed to a computer simulated
Virtual Reality) like a raytraced computer model, no matter how
properly it is cut.
Even in a tube setting, some light can and does leave thepavilion, reflect off the setting and return through the pavilion
and out of the crown for our eyes to see.
That defies the laws of physics and the basic definition of critical angle.
Maybe, but so does a bumblebee in flight. But I’m not defying the
Laws of Physics or the basic definition of Critical Angle, just
explaining that light propagation in the real world doesn’t behave
like a raytraced computer model. Physics, and Snell’s Law in
particular are wonderful for describing, in a scientific way, how
light behaves in a gemstone. But in real life you have to take into
account a lot of things besides the perfect raytraced computer
simulation. Yes, a tightly focused beam of light generally behaves
in concert with the basic description of Critical Angle and does
follow the Laws of Physics, but nobody walks around with a light
trap around the ring on their finger with a fiber optic light source
pointed at the stones. Go ahead, give it an honest try. Place the
same stone on a gold tube and a silver one. They will appear
different, however slightly.
That's why the same stone appears slightly different whencompared in different colored metals. That’s also why gem graders
and appraisers prefer unmounted stones. Most commercial CZ diamond
color grading kits are supplied with one gold color and one silver
color mounting for this very reason, to compare stones mounted in
different colored metals against the masterstones.
I suspect this has more to do with the contrast of the background OR to compensate for poorly cut stones with leakage.
You suspect wrong. Gem graders, gemologists and appraisers grade
stones with a neutral, non-UV-reflective background that is
specifically designed for that purpose. We also use overhead,
daylight-balanced fluorescent lighting that is also specifically
designed for grading. They’re called grading lamps. This is
practiced industry-wide for reasons of consistency. There are, of
course, exceptions; The Lightning Ridge Miners Association requires
all grading of Lightning Ridge Opal be done with natural light from
a window facing a particular direction, but you can see this is done
for the same reason. Also, diamond color grading masterstones are
graded by qualified gemological laboratories and are accompanied by
reports from those labs, as do CZ masterstone sets. They must comply
with a specific range of proportions, as well as where they lie in
the range of color they represent. So, a poorly cut stone with or
without proportions that allow leakage will never be certified as a
masterstone by a reputable gemological laboratory. Colored
masterstone sets are graded similarly. Additionally, stones are
typically worn in jewelry crown-up, whereas they are color graded
pavilion-up. You’re barking up the wrong tree.
There is no excuse for a leaky CZ as the material is so cheap there is no need to cheat on the angles. The cost of diamond rough motivates cutters to maximize the yield instead of getting the angles right.
Can’t argue with that, nope.
One of my previous posts mentioned that gems with pavilionangles cut less than it’s CA will allow most of the light entering
through the crown to exit the pavilion, known as Unplanned Light
Leakage, which results in pale areas of color in the stone known
as Windowing.
That is a defect in a brilliant cut stone.
Yep. That’s why it’s called Unplanned. And it’s considered a defect
in any polished stone cut, not only a Brilliant Cut. I’m not sure
why you took this excerpt out of context because it doesn’t speak to
the issue.
Well, some very dark stones are purposely cut this way totake advantage of CA to lighten it so it will have a more pleasing
look.
Right but it is no longer a brillliant cut. Dark stones absorb so much light that there is no point worrying about the critical angle because so little returns anyway. They are usually cut to present the best color and ignoring brilliance and fire.
Huh? Oh, now I see why you separated the paragraph, but you’re
absolutely wrong, anyway. A Brilliant Cut is a style of cut where
the facets are a combination of triangle and kite shapes that
radiate out from the center. That is the exact description of a
Brilliant Cut. It’s outline may be round, marquise, pendeloque (also
called pear or teardrop), triangular, square, rectangular, heart,
etc., but it is the shape of the facets that dictates whether it is
a Brilliant Cut. Proportions (such as Crown/Pavilion angles, et al)
and Windowing/Extinction (lighter or darker areas in a gem, viewed
face-up) have nothing to do with the style of cut a gemstone has, or
whether it is a Brilliant, Step or Whatever Cut. And while I agree
that there’s no point in worrying about CA as long as you know that
the pavilion angle needs to be cut shallower in a darker stone, that
is still the reason for cutting it that way.
I received my GG diploma from GIA at age 47, about 40 years after I
first became interested in rocks and gems, and about 20 years after
I began learning how to forge, fabricate, cast, facet, cab, etc. GIA
provided me with an incredible amount of and knowledge
about gems, including the natural sciences involved with their
creation and the physics involved that make them appear to our eyes
as they do. But all of the academics are tempered with real-world
knowledge and practice. In theory, physics follow absolute Laws
(well, not abstract physics so much, but you know what I mean), but
out here in the real world where it is observed (as opposed to being
theorized about), there are variables which come into play that bend
those Laws enough to where our eyes see something different from
what the Laws say we should. Academics are the best way to learn
theory, practice is the only way to observe what actually occurs.
Set a stone in a white metal, then set it in a yellow or other color
metal. The same stone, not two similar ones. It will appear
different, even if your eye isn’t trained to see it.
As a poster to this forum recently typed “In theory, there is no
difference between theory and practice. In practice, there is.” Too
true!
James in SoFl who answers the old question “How do I get to Carnegie
Hall”? Practice, man, practice.
I am not a faceter, nor am I a gemologist. It does seem to me that
when worn, faceted stones are illuminated not by a single direct
beam at a precise angle, but by diffuse, ambient light coming from
all possible directions. Given this, some of the light will strike
the pavilion inside the critical angle, some will be outside, and
there will be some degree of leakage from the pavillion.
I don’t know that this is due to the dual wave-and-particle behavior
of light to which The Doctor alludes, I think it is more the issue
of the ideal perspective of a single beam entering the stone vs the
reality of light entering the stone at all wavelengths from all
directions under normal conditions.
This would be fairly easy to test- take a well-faceted stone, place
it in a tube setting, expose it to strong but diffuse light from the
top and sides, look into the tube setting from below to view the
pavillion directly. If all you see is pitch black, then it is
established that light leakage from the pavillion under real
conditions is negligible, but if you see any light whatsoever, it is
leakage from the pavillion and the theory that no light escapes from
the pavillion is debunked.
Additionally, I think that the argument that a tube set stone should
be as bright as a prong-set stone because of zero leakage from the
pavillion, which I have heard before, overlooks the fact that with a
prong-set stone light may enter the stone directly through the
pavillion and exit through the table, which should result in a
brighter -appearing stone.
But then, what do I know…
Lee Einer
Dos Manos Jewelry
http://www.dosmanosjewelry.com
This is a fairly large post, so those of you who care nothing about Critical Angle and how light interacts with please feel free to skip this one.
You are right, it’s long but as the person you are responding to, I
did wade through it.
There is a great deal of misunderstanding on both sides here and a
great deal of info also. I think it is beyond sorting out at this
point so let me just pick one issue to clarify my original point on
angles and quantum physics is not really needed to understand it.
The issue is “windowing”, i.e. looking through the table from above
and seeing the other side of the mountain. You keep refering to
looking through the crown but that is a much more general case.
A ray, wave, quanta or line on a computer, striking the table at 90
degrees will be totally reflected if the pavilion angle is above the
critical angle. If said light strikes the crown facets at 90 degrees
to the table, they also will return out the crown somewhere but this
requires more than just above the CA because now there are additional
angles involved.
Your definition of a brilliant cut could be what the trade accepts
but it aint brilliant if the above sort of light leaks out.
It is very simple to meet the above criteria with high refractive
material by cutting the proper angles. Maximizing yield in a natural
stone is another issue and usually the one that motivates people to
rationalize windowing.
If I said or implied anything other than this, I apologize for
getting carried away. This was my only point and it is unshakeable.
js