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Refractive index matching

  1. Apr 13, 2007 #1
    How close do two refractive indices, e.g. a liquid and solid, have to be for the solid in the liquid to be invisible to the naked eye? 1st decimal place, 2nd decimal place?
  2. jcsd
  3. Apr 13, 2007 #2
    Refractive Index, cryolite

    Cryolite (Na3AlF6, sodium hexafluoroaluminate, http://en.wikipedia.org/wiki/Cryolite) has a refractive index close to water. Now it is mainly manufactured synthetically, and is therefore, I assume, pure. When I called a ceramic supply store, they had granular cryolite ca. the size of table salt xtals, and of a pure white appearance. They were happy to drop some in water, but the water simply got cloudy and as the cryolite sank to the bottom, it was still visible.
    I have noticed this with a number of granular versions of crystalline or amorphous materials, e.g. borosilicate glass shards are nearly invisible in glycerol, but borosilicate glass powder is much more visible. Likewise, "fused silica" is white, and in glycerol pales a bit but is still quite visible.
    If one assumes that two refractive indices match, can the surface characteristics of one material affect its visibility in the other, or is it purely a question of the RI's?
  4. Apr 14, 2007 #3
    Light reflected at an interface depends on the refractive indexes but also in angle of incidence.
    It happens that as angles of incidence approach 90°, that is, almost parallel to the surface the reflection tends to 1: all light is reflected. This limit is the same for all indexes.
    This means that for decent angles of incidence an immersed object with near the same index will be invisible, but for grazing incidence it will be always visible, no matter how near the two indexes are.
    If you put a sphere in a liquid, you will not see the body of the sphere, but you will see the external border. This border will thin as indexes approach and disappear only when the indexes are (almost) identical.
    To talk decimal places, the mirages you see on the road or over lakes are due to a reflection due to a difference in the fourth o fifth decimal place.
    Anyhow, this method of immersion is said to be (or have been) used to detect defaults in diamonds.
    In the case of a powder, there are a lot of angles and grazing angles. But there is also a problem of "wetability" of the grains. There can be zones where air is trapped and that will rest highly reflective.
  5. Apr 14, 2007 #4
    In other words, geometry matters. I gather that spherical objects minimize the problem of reflectance, so if cryolite, then cryolite spheres? Would be difficult to do, but perhaps in a rock polisher, at least all facets could be worn away to curved surfaces.
    It is used to visualize faults in diamonds. Diamonds are produced only at extreme depths (>150km), and their inclusions are clues to mantle chemistry. Immersing flawed diamonds in matched liquids allows visual examination of the inclusion.
    Regarding decimal places, magnesium fluoride (1.37-1.38) "disappears" in isopropanol (1.3772) (e.g. http://jchemed.chem.wisc.edu/journal/issues/2006/Aug/abs1170.html), but I have seen claims of "matching" where second, and even first decimal places did not match. This from a team of researchers trying to develop non-toxic high RI fluids for mineralogy.
  6. Apr 15, 2007 #5

    Claude Bile

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    Index matching generally refers to a reduction in refractive index contrast through the application of an intermediate substance - not necessary a complete removal of the refractive index contrast all together.

  7. Apr 16, 2007 #6
    That may be. I imagine intermediates are called for when you cannot alter the RI's of the two materials.

    This is for a photomicrography application, so appearances matter greatly. Immersion oils, for example, match the RI of the glass objective very closely.

    In my case, am photographing things in water-based media, and so need a optically clear solid in the range 1.33-1.47. One successful combination seems to be Nafion beads in water, but Nafion beads are several hundred dollars for 0.5gm, and I need ~100 gms. Tefon AF and CYTOP are also very expensive. http://aem.asm.org/cgi/content/full/71/8/4801#F2.

    The subject also comes up as a demo in a physics or chemistry class (e.g. MgF2), and is used in mineralogy to measure refractive index of crystals.

    It seems, though that the shape of the particles is also critical. Was considering photographic quality gelatin, "optically clear" silicone, fused quartz (1.45, but nobody makes small fused quartz beads!), and even hydrocellulose gels, which would not be soluble in alchohols.

    Nevertheless, the most feasible combination seems to be borosilicate glass beads in glycerin (1.474 v. 1.473). Hard to find polished sub mm sized borosilicate glass beads, however.
  8. Apr 16, 2007 #7
  9. Apr 16, 2007 #8
    I suppose that you have considered all liquids given in the Handbook of Chemistry and Physics for the "immersion method" as Quinoleine (hydroquinone) n=1.62.
    Out of curiosity: what is the index that you are looking for?
    Why water soluble? Is it living or biological material?

    If a chemical can be melted in air without changing its nature (as is the case for oxides), there is a way to transform powder particles into spheres. It is to let the power drop through a flame (hot enough) and recover the particles. Each particle melts, the surface tension transforms it in a sphere and they cool in the air before touching the recovering surface.
  10. Apr 16, 2007 #9
    I work for the Smithsonian, and we are embarking on a very large, long project to image our most valuable specimens and to make that info available on the web (reduces loan requests, diffuses knowledge...).
    These specimens are routinely stored in 75% EtOH, but can be immersed in other aqueous media for short periods, e.g. other alcohols, pure glycerine, aqueous solutions of possibly dense salts that might have high refractive indices, etc. But you can't, for example, transfer anything directly to anhydrous ethanol or xylene--might implode the specimen due to osmotic effects. I think, therefore, that pure glycerine (1.473) is about the max feasible RI, tho am open to suggestions (biologist, not a physicist!).
    They must be posed precisely. For routine viewing we use sand or like materials (e.g. maybe carborundum), but as a background for an image that is undesirable.
    Hence the desire to find optically clear, sand-like particles on which the specimens can be posed, but which will be essentially invisible. Some borosilicate glasses run as low as 1.46 (titrate glycerine w H2O...) but it has been difficult to find a source of small spheres.
    Cryolite would be ideal. Your suggestion was how they made lead shot (more or less), no? Would cryolite behave in this way? We might have the equipment to do that.
  11. Apr 16, 2007 #10
    Duh, of course cryolite could not be melted in an oxygen flame w/o change, as it is not an oxide.
  12. Apr 16, 2007 #11
    Thanks for your explanations.
    Although I knew that some said you can make lead shot this way, I was thinking about the making of synthetic sapphires. Alumina powder is dropped through a hydrogen-oxygen flame. The small droplets of alumina depose and cool on the growing crystal. I'm no chemist but I think that maybe cryolite supports heating in air.
  13. Apr 16, 2007 #12


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    So, you're looking for a "solid-liquid pair" to use for "posing" and illuminating mineral specimens, and you'd like the liquid to be "specimen friendly (no dehydration, solution of specimens)."

    Refractive indices in "common" tabulations are for the sodium "D-line" (whatever wavelength), and do NOT include dispersion at other visible wavelengths --- that's one problem. You apparently don't wish to set the specimens on a flat glass slide suspended in the liquid medium --- problems with posing?

    You might try a Saran or cellophane "hammock" or sling to support your specimens --- strikes me that those are nearly invisible in alcohols.

    You'll also want to add tenths or hundredths percent surfactant to your liquid medium to ensure wetting of both the support material and the specimen --- releases air films and bubbles from surfaces and crevices
  14. Apr 16, 2007 #13
    Solid-liquid yes, but biological, not mineralogical. Feasibility is indeed net over the gamut (literally), but haven't even got to first base.
    You are right about flat glass slides--you can't pose the specimen correctly. Sand, the classical solution, is just great for precise positioning. Saran or cellophane wouldn't compete. It's just a really inferior background for photos.
    Bubbles and wetting are not issues--all stored in 75% ethanol.
  15. Apr 16, 2007 #14
    Other high-index aqeuous combinations

    Karo syrup or sugar water (80 brix) will be up to 1.49, which will match acrylic beads you can buy at most hobby stores or Potassium Chloride crystals which you can get as "NoSalt" at the grocery store. Wet the NoSalt with a small quantity of water first in order to achieve good wetting.

    Other than being sticky, these might work for a sensitive budget and they are water-based. Check osmotic effects of these chemicals too (the syrups are only 20% H2O by weight) so you don't ruin the samples.

    Cryolite actually dissolves a little bit in water. I would avoid it because fluoride is a notoriously reactive ion, especially if you are going to do immersion photography.
  16. Apr 16, 2007 #15


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    Wondering if that wasn't the case, hence the implied question in "mineral." So, ideally, you'd like to be able to pose bugs, fish, frogs, ferns, mushrooms, and whatnot in a 75% ethyl alcohol bath between a couple flat glass plates and illuminate and change camera angles and shoot away without the subject moving, and without the exterior "armature" showing up in the picture.

    Specimen size range? Soft? Or pickled up nice and rubbery? Guessing the second --- you're talking about posing them in sand or on glass beads.

    Saran and cellophane do give you the option of cutting holes if you want legs dangling in flight, or other "action" poses.

    'Nother thing you might try is bribing the lab's pet spiders to weave webs in frames suitably sized for immersion in the studio tank --- lot more fragile than Saran or cellophane, but it sounds as though you've got fewer time constraints than the usual projects of this nature.
  17. Apr 17, 2007 #16
    It's usually a "bug" issue, as in small specimens of invertebrates and terrestrial arthropods. They range from soft to nearly indestructible. Most are <5mm, and some are < 1mm, especially the dissected parts. Because the contorted conformation of each specimen is unique, perforated saran or cellophane would be difficult. That's why a dense medium like sand works well. Barely submerged or nestled, the specimen holds still.
    I tried fructose in a different context, as a stiff gel that might suspend the specimen long enough to take the pic, but no. Things fall down, constantly, and exposure times are too long.
    Fructose had the problem that the ethanol:H2O layer on specimens was quite apparent, tho I suppose fructose and ethanol are eventually miscible.
    Nevertheless, fructose:plastics is a good suggestion--will keep it in mind.
    Actually, time is money; we will spend, eventually, millions doing this. The photo systems alone run 30-80K. We optically section the specimen using automated z-stepper stages, then assemble the stacks of photos into one composite image. Check out http://helicon.com.ua/pages/focus_samples.html for a simpler version of the process. 5-10 images per stack, and three views per specimen--that's a lot of time.
    Re fluoride reactivity, that is certainly a concern--as is the overall toxicity of the components. Obviously, will experiment with worthless stuff first. The Australian publication used concentrated potassium thiocyanate, and Cargill refractive index fluids >1.5 get extremely toxic quite quickly.
    Thanks to everyone for their help. It is great to be able to draw on other's knowledge. Have another question about the intrinsic nature of photography in fluids, which will stick in another post.
  18. Apr 17, 2007 #17
    Fundamental photo question:
    Have noticed that, technical problems aside, photos in liquid are never on average as sharp as photos in air. Is this due to the RI of air v. e.g. water? If things are ca. 1.5, photo'ed in 1.00 air, that's quite a diff. Photo'ed in 1.33 water, less diff. If things disappear when RI's are equal, does it follow that things are most visible when RI diff is max?
    NB. I realize the issues of RI transition between glass of lens to air to water surface, oil-immersion optics, etc., but am wondering if, e.g. we all lived underwater, whether the brightness/clarity/brillance of our visual field would be less because it was immersed in 1.33 v 1.00?
  19. Apr 17, 2007 #18
    I suppose that we don't talk about reflections at the interface water-air.
    I suppose that the water is crystal clear.
    I suppose that there are not ripples at all on the water surface.
    Then I do not see any reason why the center would be less sharp than in the air.
    I do see a reason for the image to be out of focus outside the center. Because of the refraction at the interface for non perpendicular rays, the shape of the surface in focus is not a plane but a spherical surface.
    You can see this phenomenon when you look at the rear wall of an aquarium: it does not seem to be a plane.
    Bu if you work at small apertures and with all angles small, this should not blurry the image.
    When you open your eyes under the water, you can not focus because the optical power of the cornea is less than in the air. But brightness/clarity/brillance are the same (not in real water but on NASA water, yes).
    Have you tried to put an optical plane glass over the water surface to avoid any ripples?
    Do the samples ooze other RI liquids in the water?
  20. Apr 17, 2007 #19
    Have tried placing slides a/o cover slips on top, obviously w/o bubbles, etc., but it does not markedly improve the image. To some extent it may also be a question of mag...high mag photos (~30-60x) are always a little softer. But it is still my impression that photos through liquid are inferior to photos in air, all other things being equal.
  21. Apr 17, 2007 #20


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    As the refractive index of the medium increases, the effective focal length of the lense increases; you're trying to focus at the same distance, and losing depth of field; you could "stop it down" further, but sounds like you've already hit the limit on that. Also aggravates chromatic aberrations.

    Getting back to the refractive index question, something to play with (no results guaranteed) is picking a handful of denser polymers, in suitable bead sizes, that are not soluble in water-ethanol (PVC, TFE, whatever else you can come up with), and run them through several boiling-cooling cycles in the water-ethanol to get rid of as much of the air "dissolved" in the beads as is convenient (that's gotta contribute to some of the "visibility" problems), and you may get lucky (little surfactant would definitely help here --- plus, if you do get lucky, you'll want to keep your bead bed wet). PVC, TFE, PE, PP are all available in varieties of AMWs (chain lengths) and densities --- hit the suppliers with the "Smithsonian" line, and they may be willing to do things for the PR value (or tax write-offs).
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