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Why does repeated reflection increase amplitude?

  1. Sep 16, 2011 #1
    diamonds shine brightly because light inside them is reflected repeatedly within its walls.why this repeated reflection increase the brightness of light??
  2. jcsd
  3. Sep 16, 2011 #2
    I've done a lot of faceting and I'm not sure what you mean by "increase the brightness". You are not ever going to get more light (more photons) out of a diamond than went into it in the first place. The apparent increase in brightness comes about in several ways:

    1. Cutting and polishing increases the ability of the raw material to pass light in through the surface layer. Rough diamonds are pretty dull things. This polishing also increases surface reflection, which makes the stone appear brighter.

    2. The faceting angles reflect much of the transmitted light back out through the crown instead of allowing it to pass down and out through the pavilion. This does not take very many reflections--three or four at most,according to most cutting diagrams. The laws of optics suggest that each reflection diminishes the intensity of the light. A reflection does not increase the intensity. Increasing the intensity would involve adding photons to the beam.

    3. The angle at which you view the stone has a tremendous effect upon its apparent brightness. Jewelers know this and carefully positions their stones in their displays so as to get the best effect.

    Even very fine stones cut with excellent precision and highly polished reflect less light out of the crown than enters into it.
  4. Sep 16, 2011 #3


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    The very short answer is that judicious design of reflection optics causes more light to return to the viewer than would if it were not cut that way.

    i.e. cut, it might return 80% of incident light back to the viewer, comared to uncut, it might only return 20% (totally arbitrary numbers).
  5. Sep 17, 2011 #4
    thank you very much!!
    but can you tell me why diamond sparkles more than glass??is it simply because glass absorbs more light?what role does the refractive index of diamond have in this?
  6. Sep 17, 2011 #5


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    Same thing. The higher refractive index means it will reflect more light back toward the observer - internal reflection is more efficient, even at lower angles. Think of reflection on water at a high angle (more perpendicular) versus at a low angle (more oblique). A higher RI means the diamond will reflect internally even at a high (more normal) angle.
  7. Sep 17, 2011 #6

    Ken G

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    Perhaps a point of clarification would be to say that the light has to go somewhere, so even if there is no absorption anywhere, if a diamond shines more brightly than glass in some directions, then it must shine less brightly in others. Glass is often built to pass light through, diamonds are often built to reflect light back, but especially from certain places in certain directions. klimatos could probably tell us a lot about the clever ways that diamonds are faceted, but it would seem to me that the basic idea behind a "facet" is to have a small region of the diamond emit a lot of light in a given direction, rather than all the diamond emit dimly in all directions. The high index of refraction and high reflectivity allows it to take full advantage of the facet concept.
  8. Sep 17, 2011 #7
    Thanks, Ken.

    In a nutshell, the basic purpose of a facet is to reflect incident light rather than pass it through. The higher the refractive index, the easier and more effective this is. The standard "brilliant" cut has sixty-five facets, of which the table (the flat top of the crown) is the largest. Facet angles are functions of the refractive index of the material, and are designed to reflect the maximum amount of incident light and transmit the minimum amount.
  9. Sep 19, 2011 #8
    Thanks a lot
  10. Sep 19, 2011 #9
    Would I merely be confusing things to suggest the possibility that diamonds, being crystalline carbon, are more translucent than amorphous glass (to me, there's no such thing as an amorphous solid, so glass is a liquid!), and that, being a translucent crystalline solid, as opposed to a liquid, the light reflected from the interior of a diamond and refracted through the facets and edges is going to be more organized, and, therefor, (particularly when added to the reflection back from the facets and edges) more brilliant to the eye, than that same light passing through amorphous glass?
    Last edited: Sep 19, 2011
  11. Sep 19, 2011 #10
    I really have no idea as to whether glass or diamonds absorb more light, but my guess is that glass, being amorphous, allows the light that enters it to leave in no particular direction, whereas the crystalline diamond is simply better at reflecting light, whether from the facets or from the facet joints, back with an organized directionality that makes it appear to be brighter than glass. The superior and more subtle prismatic effects of refraction of light through the facets and facet-joints of diamonds of what little light may actually be reflected from the interior (or passed through from the side of the diamond opposite the viewer) are what constitute the "fire" of a diamond, which optical effect simply cannot be recreated with amorphous glass.

    This being said, I must most emphatically state that white diamonds are much too common a material to be worthy of their current retail market value.
    Last edited: Sep 19, 2011
  12. Sep 19, 2011 #11

    It just so happens that I own a white diamond, and I've just observed it and noted its optical characteristics.

    It's a European Gemological Institute-graded 0.71 carat Round Brilliant; Polish: Very Good; Symmetry: Good; Clarity Grade: S12 (midrange of "Slightly Included"); Color Grade: G (Near-Colorless Rare White Wesselton); non-fluorescent.

    Having observed it carefully both under light and with light passing through it, I can definitely say that the fire from the crown is much more brilliant with the crown under the light than it is with the light passing through the length of the diamond, but there's a lot of brilliance passing through the length of the diamond when you hold the crown up to the light and view it throughout the pavilion. When you hold the crown up to the light and view the diamond through the pavilion, most of the light seems to reach my eye from the facets, but when I hold the pavilion up to the light and view the diamond through the crown, very little brilliance is observable. The opposite happens when I hold the diamond under the light: the crown is brilliant with fire (almost all of which seems to come from the facet edges), and the pavilion seems much less brilliant.

    All of this suggests to me that there's a very complex interplay going on here between reflection from the facets, reflection from the facet joints, refraction from the interior through the facets or the facet joints, and reflection from the opposite facets or facet joints (might one call such reflection "refraction-resistive reflection?).

    I totally agree with you that the bulk of the fire from the crown seems, according to these observations, to derive from superficial reflection, supported by light returned to the eye by other factors of reflection and refraction.
    Last edited: Sep 19, 2011
  13. Sep 19, 2011 #12
    Passing Point of Information:

    Most stones that a jeweler would describe as "glass" are actually rock crystal. This is the crystalline form of silicon dioxide and is not amorphous.
  14. Sep 19, 2011 #13
    I honestly didn't know that.

    I thought that, in the context of this discussion, "glass" meant "glass".

    So sorry!


    By the way, what do you think of my rock?

    More importantly, what do you think of my analysis of the optical properties of diamonds?
    Last edited: Sep 19, 2011
  15. Sep 19, 2011 #14
    No need to be sorry. I learn something new every time I log on.

    As I understand it, stones with high refractive indices reflect more light and transmit less light than stones with low refractive indices. Hence, they are more "brilliant".

    I think you have a nice stone, although I have no experience cutting diamonds. However, except for the exceptional color, your stone would not draw more than passing attention from most diamond dealers.
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