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.
© Optical Society of America
Iridescence is a well known natural color phenomenon. It is can be found in bird feathers, the bodies of some insects, and even some plants [1, 2, 3]. Iridescence is mostly attributed to an interference effect caused by optical interference coatings. Many shells show beautiful iridescence colors. A polished shell of mollusk Pinctada Margaritifera from the Tuamotu Archipelago of French Polynesia shows very strong iridescence colors (Figure 1). This shell exhibited little iridescence before being polished. Pearls and mother-of-pearl also display this phenomenon, but usually less pronounced. In gemology, this optical phenomenon displayed by pearls is generally referred to as “orient.” The iridescence color of the objects varies with changes in both the angle of incident light and the angle of observation. The iridescence color of this shell from French Polynesia does not exhibit the color contour lines usually seen in the color pattern caused by interference. Rather, the same color appears over a very large area, regardless of a thickness change of the shell.
Brewster may have been the first to study this phenomenon in the 19th century. He attributed the iridescence color to diffraction caused by grooves on the surface of a shell. Since the groove density of most mother-of-pearl is relatively low for visible light, Pfund used red and infrared light to study this diffraction, and obtained a diffraction pattern. Rayleigh studied the groove structure of mother-of-pearl, and regarded it as a Michelson’s echelon grating. Raman and Krishnamurti produced several photographs of the diffraction spectra of the iridescence color of shells. Since the nacre of a pearl has a layer structure which may cause interference, the iridescence color has been attributed to a combined effect of both interference by the layers of the nacre, and diffraction by the groove structure[8, 9]. This phenomenon has also been attributed to interference only[3, 10]. At present, the explanation that iridescence color is caused by interference seems to be more widely accepted.
In this paper, we studied a shell from the Tuamotu Archipelago of French Polynesia by using a SEM, and by an optical experiment, to determine the cause of the iridescence. The conclusions we obtained should end the long-time argument about the cause of the iridescence of shells and pearls. Our study may also suggest that the previous explanations about the iridescence in nature may need to be re-examined.
2. Surface structures of the shell
Although many shells show iridescence color, this shell shows the strongest iridescence color we have observed so far. SEM pictures of this particular shell were taken at different locations to study the surface structure (Figure 2).
The thickness of the nacre layer varies with different kinds of shells. In the studied shell, it is approximately 0.4 μm. After polishing, the outer surface of the shell exhibits a fine-scale groove structure (Figure 2A). The width of each groove is about 3.38 μm. Thus, this shell is equivalent to a 296 grooves-per-millimeter (grooves/mm) reflection diffraction grating. The parallel grooves are well arranged, and the surfaces of the grooves are relatively smooth and even. At such a groove density, the shell surface functions very well as a reflection grating. The diffraction color is very strong (again, see Figure 1). This implies that the iridescence color of polished shells, as well as pearls and mother-of-pearl, is caused by diffraction.
The iridescence color of the polished inner surface of the shell is much less intense than that of the polished outside surface. To investigate the reason why it is weak, an SEM picture was also taken from the inner surface (Figure 2B). By comparing the SEM images in Figure 2A and 2B, the grooves of the inner surface are more widely separated than those of the outside surface. The width of each groove of the inner surface is about 11.5 μm. This inner surface functions like a 87 grooves/mm diffraction grating. A diffraction grating with such a low groove density cannot produce strong diffraction in the visible range; therefore, it can only produce a weak iridescence color. Moreover, the surface of each groove is very rough, which also causes the inner surface to produce less diffraction. For these reasons, the iridescence color produced by the inner surface is much weaker than that of the outside surface. When the width of the groove is much greater than 11.5 μm, that portion of the shell does not show any color. This also suggests that the shell does not produce an interference color, since a larger smooth surface should produce a strong interference color, but this is not the case.
When viewed with an SEM, each layer of the nacre consists of irregular polygonal tiles of crystalline aragonite (Figure 3). The aragonite tiles are mortared together by polysaccharide and protein fibers called conchiolin. The conchiolin itself forms a matrix with irregular polygonal cells. Each layer of nacre is considered as being optically non-uniform. The nacre of our shell also shows a similar non-uniform structure.
When a beam of light is incident into the nacre, it is strongly diffused by the layers. Thus, pearls normally appear a milky white without any iridescence color. Diffused light cannot cause interference; therefore, the layer structure of pearls and shells cannot produce the iridescence color. On the other hand, if the layer structure could produce interference, any pearl would show the iridescence color phenomenon. Although the diffused light contributes little to the iridescence color, it does contribute to the bodycolor which represents the overall appearance color of shells. Our shell has a very dark brown (black) bodycolor at the area showing the iridescence color.
3. Diffraction pattern caused by the shell
The diffraction effect of this shell was verified by an optical experiment using a laser. Figure 4A illustrates the experimental arrangement with a diffraction image produced by this shell. This image is a typical diffraction pattern produced by a reflection grating (see Figure 4B). Diffraction maxima from order -2 to order 2 can be easily seen. Most diffracted light is concentrated in the higher orders, which form the large bright area in the Figure 4B. By carefully adjusting the position of the laser beam on the shell, a detailed diffraction pattern was obtained (see Figure 5). The bright area consists of the diffraction orders 3 to 8, although the order 3 is not clear. These higher orders produce the iridescence color; other orders contribute a very little to it.
The slightly curved surface structure of the shell is far from being a perfect optical quality reflection grating. The surfaces of the grooves are also slightly curved. When a light beam is incident on the surface, both surface reflection and diffraction occur. The light intensity at the zero order position is very intense; however, most of the light comes from the mirror surface reflection. When a light is directly incident on the shell, (for example under direct sunlight), and observed from the reflection direction, the shell does not show the iridescence color.
The intensity of the iridescence color of shells is directly related to the groove density of the diffraction grating of the shell. A shell with a diffraction grating structure at a groove density of 300 grooves/mm (Figure 2A) can show a very intense iridescence color (such as that in Figure 1). When the groove density is reduced to about 100 grooves/mm, the iridescence color becomes much weaker. Most shells show a weaker iridescence color, and their groove densities are less than about 87 grooves/mm (Figure 2B). The structure of each groove also affects the efficiency of diffraction produced by the shell, which in turn affects the intensity of the iridescence color. For a high groove density, if the grooves are smooth and evenly distributed, the shell will produce a strong iridescence color, and vice versa.
4. Color appearance under a microscope
Although this shell shows the same color over a large surface area, a rainbow-like spectrum can be observed under a microscope (Figure 6). Because the width variation of the grooves and their roughness, and the convex shape of the shell, the diffraction orders in the bright area usually cannot be distinguished. Rather, they overlap to produce the rainbow-like spectrum, which is the iridescence color.
This study clarifies some earlier ideas about what causes the iridescence color of shells. It is due to diffraction caused by the reflection grating structure of shell. Our experiment with a laser clearly demonstrates this behavior. No evidence of interference was found. In fact, the iridescence color is produced by an overlap of the light from several orders of diffraction. The natural structures of shells are usually not in the form of a reflection grating. Therefore, most shells do not show the iridescence color. The strength of the iridescence color depends on the groove density and the surface quality. A shell with high groove density, and with a smooth and even surface can produce a strong interference color.
Our recent study of “orient” of pearls, which is the iridescence color on the surface of a pearl, also finds that the grating structure on the surface of a pearl causing this phenomenon. High density and smooth grating structure causes a strong orient. A pearl with a random surface structure shows no iridescence color.
We thank Mr. B. Wan for providing the shells for this study, Dr. P.D. Yang and Dr. P.Y. Feng for taking the SEM pictures, Dr. K. Nassau for his suggestion using a laser to prove diffraction, Dr. M. Johnson for her comments on the manuscript, Mr. G. Ravich for his help to take the diffraction pictures, and Mr. D. MacDonald for loaning the laser.
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