Biomedical Engineering Reference
In-Depth Information
The males of two butterfly species,
Lampro-
lenis nitida
from New Guinea and
Pierella luna
from Central America, display intriguing irides-
cence of the forewings due to diffraction
[73, 74]
.
The forewings appear matte brown when illu-
minated from above but show rainbow colora-
tion when illuminated along the axis from the
body to the wing tip. The reflection changes
dramatically in hue with the viewing angle.
Males of the
L. nitida
species display green to red
color when illuminated from the front and dis-
play blue to violet when illuminated from the
back. This pleochroism occurs because the fore-
wings contain two separate photonic structures
in each scale, each of which causes a separate
iridescent signal of different colors that can be
seen in different directions
[73]
. The forewing of
this butterfly decomposes white light, just as a
diffraction grating would do. The cross-ribs of
its scales, shown in
Figure 11.15
, can be viewed
as a diffraction grating with a roughly 580-nm
step lying nearly flat along the wing surface. A
white beam at normal incidence diffracts, and
the red diffracted beam emerges at grazing
angle, whereas orange, yellow, and green dif-
fracted beams appear at smaller viewing angles,
increasingly closer to the vertical direction. The
color sequence in
P. luna
is reversed, compared
to
L. nitida
. Violet light exits at grazing angle,
near the forewing surface; the other colors, from
blue to red, emerge at viewing angles progres-
sively closer to the direction perpendicular to
the wing
[74]
. This effect is due to a macroscopic
deformation of the entire scale, which curls to
form a grating perpendicular to the wing sur-
face, and functions in transmission but not in
reflection
[74]
.
Flashing green iridescent light from seed
shrimps, which are crustaceans of class
Ostra-
coda
, can be also explained with the diffraction-
grating effect. The reflecting surface of a seed
shrimp contains fine parallel grooves similar to
those on a compact disk. These grooves are used
to provide a visual display during courtship
[42]
. Surface structures shaped like diffraction
gratings have also been found in the fossils of
Burgess Shale species, such as of the genuses
Canadia
and
Wiwaxia
, living 515 million years
ago, and must have given rise to radiant colora-
tion
[42, 75]
.
11.4.3 Structural Colors Due
to Collaborative Effects
In general, a structural color changes its hues
according to the viewing angle. The glittering
blue of South America's
Morpho
butterflies, on the
contrary, exhibits a weak angular dependence,
indicating that more than one physical mecha-
nism must be responsible. Although pigment
granules (mainly biopterin) have been found in
the scales of some
Morpho
species, their quantity
is insufficient to explain the deep radiant blues
[5, 76]
. Instead, these colors result from a collabo-
ration of multilayer interference, the diffraction-
grating effect, and the non uniform heights of
the ridges on the scales on the wings
[27, 28]
. The
iridescent blue and purple colors of the wings of
Hypolimnas anomala
, a Southeast Asian butterfly,
also arise from complex microscopic structures
on the surface of the scales.
FIGURE 11.15
SEM of a wing of the butterfly
Lampro-
lenis nitida
.
(Source:
National History Museum, London, UK;
