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Shell surface creates iridescent colors

25 Oct 2004

Researchers reveal the origins of the blue-green and pink iridescence seen from the shell of a common mollusk.

Both diffraction and interference effects contribute to the vivid blue-green and pink iridescent colors seen from the shell of the mollusk Haliotis Glabra. That's the conclusion of a team from the Nanyang Technological University in Singapore. (Optics Express 12 4847)

Haliotis Glabra is native to the Philippines. Like many pearl and mother-of-pearl materials, the color of the shell under white light varies with the angle of observation.

Augustine Tan and colleagues were curious to find out the mechanisms that combined to produce the shell's strong iridescent colors. The origins of the colors were examined by studying both the microstructure of the surface and cross-section of the shell.

The researchers used a He-Ne laser emitting at 632.8 nm and a scanning electron microscope (SEM) to study the surface of the shell. Strong diffraction patterns revealed two sets of grooves: one close-spaced set of between 2 to 8 microns in width and a wider-spaced set of around 30 to 50 microns in width. These observations were confirmed using SEM images.

Fourier transform infrared spectroscopy (FTIR) combined with SEM images unveiled uniform stacks of nacreous layers just below the surface of the shell. According to Tan, the presence of such regular stacks of thin layers strongly suggests that multilayer interference plays an important role in generating the iridescent colors.

"The surface of the shell has a fine-scale diffraction grating structure and stacks of thin crystalline nacreous layers are found below the surface," say Tan and co-workers in their paper. "These observations suggest that the iridescent colors are caused by both diffraction and interference."

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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