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Opal Physics

#1

Anyone, Just wondering about opal again. I have heard many times of
the fact that opal colors stem from destructive interference from
the orderly array of hydrated silica spheres and voids like
diffraction grating. Now, what of the very high quality opals that
seem to “glow in the dark” with color? You know, the intense red
flash in a room with only one candle and otherwise total darkness?
Is it simply that our pupils have dialated to accomodate the
darkness? If the opal uses destructive interference or simple
diffraction, you cannot have more light photons exiting the stone
than entering at a given wavelength. Could it be that the silica
spheres are actually stacked not precisely in a plane, but rather
some slight parabolic shape that could in some way concentrate a
larger area of a given photon density into a higher photon density
but a smaller cross section?

Any thoughts?

Blaine

#2 
 Could it be that the silica spheres are actually stacked not
precisely in a plane, but rather some slight parabolic shape that
could in some way concentrate a larger area of a given photon
density into a higher photon density but a smaller cross section?

No. If they did that, they’d no longer properly act as a
diffraction grating to give you single color reflections. these very
fine opals are simply those where the size of the spheres and their
arrangement is most uniform, giving the most effective selection of
the wavelengths, and least loss of light at that wavelength.
Remember that the destructive interference is destroying the colors
you DON’T see. What you see is being quite effectively reflected.
And yes, your ability to see it in dim light is the fact that your
pupils have now opened up and are seeing more light. However, there
is at least a little something in your reasoning that’s worth
remembering. The curved front and back of an opal cabochon can act
somewhat as a lens. Cab cut stones often are a little brighter or
more optically interesting than those cut with flat tops and backs.

   Any thoughts?

Don’t forget Occams razor. Given all possibilities equal, the
simplest is likely to be true… In this case, there’s no reason to
complicate matters. The simple stated structure of opal is quite
capable of displaying the effects seen.

Peter

#3

Hello Blaine, Try this, take a very small piece of mirror into a dark
room. At one end of the room light a candle and then stand at the
other end with the small piece of mirror. Rotate the mirror until you
can see a reflection of the candle within it. Notice that the
’brightness’ of the candle and of the reflection are almost
identical. Now try this with a CD (music or software). The CD acts
similar to a diffraction grating and to certain layers of silica
spheres within opal. Very little loss of light here. Hope this
helps. Will Estavillo

#4

Hi Peter Perhaps, this is the exception that proves the rule, but I
cut a matrix opal material from Queensland that is more lively
(displays more color) when it is cut flat. I orient the material
very precisely, cut the table first, then cut the rest. It also
stikes me that this material shows the brightest color when the stone
is seen off to the side of the source of light. I don’t know of
anything like it. KPK

#5

I’ll try to draw you a “word picture.” I have seen an electron
microscope photo of opal. If you visualize a surface covered with
spheres (like ping-pong balls) they are ‘packed’ in synmmetrical,
even rows in both directions. If you then add a second, third, and
fourth, etc. layer, EACH layer is offset slightly, by the dimension
of the radius of the sphere, so that the curves of the spheres of
layer #2, etc. fit into the space between the spheres of layer #1,
and so on. Does that explanation make sense? That is the way opal
molecules(???) line up in the photo. David, Lord of the Rings