Abstract

Shells and pearls often show iridescence color. The cause of this phenomenon has been attributed to diffraction, both diffraction and interference, or interference alone. We used a shell of the mollusk Pinctada Margaritifera, which shows very strong iridescence colors, to study how this color is produced in the layers of nacre in shells. From observations with a scanning electron microscope (SEM), this particular shell exhibits a very fine-scale diffraction grating structure. This suggests that the iridescence color is caused by diffraction, which was demonstrated by an experiment using an argon ion laser illuminating the shell to produce a distinct diffraction image. The strength of the iridescence color can be correlated to both the groove density of the diffraction grating formed by the shell, and the surface quality of the grooves themselves. A shell with a high groove density and a smooth groove surface produces a strong iridescence color.

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References

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  1. J. Turner-Valle, "Optical interference coatings in nature," Opt. Photon. News 9, 58-59(1998).
    [CrossRef]
  2. D. W. Lee, "Iridescent blue plants," Am. Sci. 85, 56-63(1997).
  3. K. Nassau, The Physics and Chemistry of Color, (Wiley, New York, 1983), pp.265-268.
  4. D. Brewster, Treatise on Optics (1853), pp. 137-149.
  5. A. Pfund, "The colors of mother-of-pearl," J. Franklin Inst. 103, 453-464 (1917).
    [CrossRef]
  6. F. Rayleigh, "Studies of iridescent colour, and the structure producing it. -- II. Mother of pearl," Royal Society of London Proceedings A 102, 673-677 (1923).
  7. C. Raman and D. Krishnamurti, "The structure of optical behaviour of pearls," Proceedings Indian Academy of Sciences 39A, 215-222 (1954).
  8. A. Alexander and H. Sherwood, "Gemmology for Beginners," Gemmologist 10, 48-52 (1940).
  9. H. Smith, Gemstones (Methuen, London, 11 th ed., 1944), p. 440.
  10. E. Fritsch and G. Rossman, "An update on color in gems. Part 3: Colors caused by band gaps and physical phenomena," Gems & Gemology 24, 81-102(1988).
    [CrossRef]
  11. C. Raman and D. Krishnamurti, "On the chromatic diffusion halo and other optical effects exhibited by pearls," Proceedings Indian Academy of Sciences 39A, 265-271 (1954).
  12. P. Reitz and P. Juergens, "Pearl colors," Gems & Gemology 8, 139-140 (1941).

Other (12)

J. Turner-Valle, "Optical interference coatings in nature," Opt. Photon. News 9, 58-59(1998).
[CrossRef]

D. W. Lee, "Iridescent blue plants," Am. Sci. 85, 56-63(1997).

K. Nassau, The Physics and Chemistry of Color, (Wiley, New York, 1983), pp.265-268.

D. Brewster, Treatise on Optics (1853), pp. 137-149.

A. Pfund, "The colors of mother-of-pearl," J. Franklin Inst. 103, 453-464 (1917).
[CrossRef]

F. Rayleigh, "Studies of iridescent colour, and the structure producing it. -- II. Mother of pearl," Royal Society of London Proceedings A 102, 673-677 (1923).

C. Raman and D. Krishnamurti, "The structure of optical behaviour of pearls," Proceedings Indian Academy of Sciences 39A, 215-222 (1954).

A. Alexander and H. Sherwood, "Gemmology for Beginners," Gemmologist 10, 48-52 (1940).

H. Smith, Gemstones (Methuen, London, 11 th ed., 1944), p. 440.

E. Fritsch and G. Rossman, "An update on color in gems. Part 3: Colors caused by band gaps and physical phenomena," Gems & Gemology 24, 81-102(1988).
[CrossRef]

C. Raman and D. Krishnamurti, "On the chromatic diffusion halo and other optical effects exhibited by pearls," Proceedings Indian Academy of Sciences 39A, 265-271 (1954).

P. Reitz and P. Juergens, "Pearl colors," Gems & Gemology 8, 139-140 (1941).

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Figures (6)

Figure 1.
Figure 1.

The iridescence color of a polished shell of the mollusk Pinctada Margaritifera from the Tuamotu Archipelago of French Polynesia. The strength of the iridescence color displayed by this shell is exceptional.

Figure 2.
Figure 2.

The reflecting grating structures of the shell as observed by SEM. (A). The grooves are arranged in parallel on the outer surface. The widths of the grooves are about 3.38 μm. The grooves form a very efficient reflecting grating to produce the iridescence color. X1,800. (B). The reflecting grating structure of the inner surface of the shell at an approximately similar magnification as (A). The width of the grooves is on average about 11.5 μm. The grooves are very rough. X1,700.

Figure 3.
Figure 3.

The irregular polygonal tiles of crystalline aragonite of the shell.

Figure 4.
Figure 4.

Diffraction images produced by the shell. (A) Optical arrangement for testing the diffraction caused by the shell. An argon ion laser was used to provide a green light at 514.5 nm wavelength. The laser beam is directly incident on a piece of the shell. The diffraction pattern is formed on the screen. (B). The diffraction pattern produced by the shell. Most diffracted light is concentrated in the bright area. From left to right, diffraction maxima from order -2 to order 2, and the higher orders are overlapped.

Figure 5.
Figure 5.

A detailed diffraction pattern produced by the shell. Only when laser beam is incident on the shell area where the grating structure is very fine, can such a detailed diffraction be obtained. From left to right, diffraction maxima form order 0 to order 8.

Figure 6.
Figure 6.

The rainbow-like spectrum produced by the reflection grating structure of the shell as seen with a microscope. X40

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