1954 Chemical Reviews, 1999, Vol. 99, No.

7 Srinivasarao

Such variations can be accomplished either by section, we will discuss some of the results that have
producing materials with different refractive indices been reviewed by Neville and Caveney,5,7 pointing out
for every color or by noting that different colors can the ingenious ways that nature uses such structures
be produced by making the interference films from for a variety of purposes. I will primarily discuss only
two different materials, one of high and the other of the color aspects of this topic. The structure of the
low refractive index, and varying the average index exocuticle has other implications, which will not be
simply by varying the composition of the two sub- addressed here, but the interested reader is referred
stances. It is rather ingenious of nature to produce to an excellent monograph by Neville.7
the variety of colors found simply by varying the Many species of beetles, all Scarabaeidae, were
proportion of the two substances, air and keratin. examined5 in a simple, yet brilliant study of the
This is demonstrated in Figure 30 which shows the optical properties of these beetles. One of the first
observations was that these metallic-looking beetles
reflected circularly polarized light, and in most cases
studied, the reflected light was left circularly polar-
ized. This can be studied using right circular analyz-
ers which extinguish left circularly polarized light.
When the beetles were viewed with right circular
analyzers, the beetles appear dark with no color.
When the beetles were viewed at increasing angles
of incidence, the colors of the cuticle shifted to lower
wavelengths, as observed by many earlier research-
ers.73,74,81 This is a clear indication that the colors are
due to the phenomenon of interference. We have
already dealt with such interference colors in detail.
However, reflectivity from the structures discussed
in the earlier part of the paper does not lead to
circular polarization of the reflected beam. Neville
and Caveney also made observations of the light
transmitted by the exocuticle and found that the
transmitted light had a color different from the
reflected colors and that the transmitted light was
also circularly polarized but in the opposite direction.
In other words, the colors in transmission did not
Figure 30. Cross section of iridescent feather surface at correspond to colors due to birefringent objects with
high magnification (Clytolaema Ruricauda, red gorget). uniaxial symmetry, since such materials would ex-
(Reprinted with permission from ref 75. Copyright 1960 tinguish light under crossed polarizers on rotation.
Optical Society of America.) Onslow, in 1921, suggested that the layered struc-
tures responsible for the colors are located in a
section of a platelet taken using an electron micro- surface layer of λ/2 thickness.82 The experimental
graph. The platelets consists of tiny air pockets results of Neville and Caveney5 clearly demonstrate
embedded in keratin with as many as eight layers that such an interpretation is quite incorrect. In
forming them. This structure is responsible for the working with several species of beetles, Caveney
thin-film interference and the color of the hum- found an exception to the rule of all the beetles
mingbird feathers. The measured reflectivity curves reflecting only left circularly polarized light. For the
agree with the calculated curves. Details of the species, P. resplendens, the reflected light consisted
optical model can be found elsewhere.75 Many of the of both left circular and right circular polarization.6
iridescent bird feathers have similar morphologies The beetle, gold in color, has a peak reflectance at
which produce color. The simplicity with which colors around 560 nm for left circularly polarized light and
are produced in bird feathers is simply astounding. a broad reflectance peak for right circularly polarized
light between 575 and 624 nm, with little reflectance
V. Color of Beetles in the blue part of the visible spectrum. Much of this
optical behavior of the beetles is due to the (solidified)
A. Color of Scarabaeid Beetle Exocuticle: cholesteric nature of the exocuticle. Therefore, we
Selective Reflection first discuss the properties of a cholesteric phase and
Neville and Caveney5-7 studied the colors of many return to the reflectivity of beetles.
beetles and found that the exocuticles of these beetles Of the many thousands of organic compounds that
had remarkable optical properties, such as selective have been synthesized, a significant fraction exhibit
reflection of left circularly polarized light, high optical a liquid-crystalline phase.79,80 Liquid crystals are a
rotation of transmitted light, and a brilliant metallic state of matter that have order between an isotropic
appearance. All of these optical properties bear liquid and a crystalline solid. These are fluid phases
remarkable similarity to the optical properties of but possess molecular order, leading to some unusual
cholesteric liquid crystals (CLCs), which have been physical and optical properties. Depending on the
studied extensively76-78 since the discovery of liquid- nature of molecular ordering, these phases can be
crystalline phases.79,80 Neville and Caveney5 con- classified as nematic, cholesteric, or smectic phases.79,80
cluded that the exocuticles behaved as optical ana- In the case of a cholesteric liquid crystal, the rodlike
logues of the cholesteric liquid crystals. In this molecules that comprise this fluid phase have long-