A new hue
As we come to understand the secrets of iridescence in
nature, we can harness this colourful phenomenon for
a more sustainable future, finds Michael Allen
FL
AFABR
I/A
LA
MYY
OU dip a plastic wand into a mixture of
water and washing up liquid and raise
it to your lips. With a gentle blow, you
unleash a stream of bubbles that bob about in
the air, winking a swirling rainbow of colours
back at you as they reflect the light. This is the
beauty of iridescence.
You can also see it on the wings of
birds and the shells of beetles. It is a completely
different way of creating colour than the
pigments and dyes we typically load onto
brushes and into printers, one that is more
subtle and adaptable. Now, we are starting to
get a deeper understanding of iridescence.
We are learning that animals make use of it
in surprisingly varied ways, and it seems we
could soon join them in harnessing this
phenomenon to pull a few tricks of our own.
Most of us think we know the basics of how
colour works. A ray of white light, composed
of many different wavelengths or colours,
strikes an object. Some of the wavelengths
are absorbed by pigment molecules and the
More recently, we have learned that
surface structures don’t always produce
that characteristic shimmering iridescence.
Tiny structures on surfaces can also produce
more ordinary looking colours – and they
can be some of the darkest and brightest
in the natural world.
In 2014, biochemist Silvia Vignolini at the
University of Cambridge and her colleagues
showed that the brilliant white Cyphochilus
beetle from South-East Asia uses this structural
effect to achieve its colour. Their scales scatter
and reflect light more efficiently than any
other known biological material. Since these
beetles like to hide among a certain kind of
white fungi, it isn’t difficult to see why an
ultra-white shell is handy. The secret of the
insect’s dazzling look seems to be that the
tiny rods on the scales are of a very particular
size. “We know that if structures are similar
in size to the wavelength of light, then they
can scatter the light,” says Qingchen Shen,
who is based at Vignolini’s lab.remaining ones are bounced back and seen
as a particular colour. All this is true – but it
isn’t quite the full story.
Colour can also be produced by surfaces
that reflect, or scatter, different wavelengths
of light back in slightly different directions.
This is what happens when we look at the
surface of a bubble or a bird’s wing and see
that characteristic iridescent shimmer. If you
move your eyes, the colour of the surface
seems to shift and dance. The reflections are
caused by tiny structures – bumps, hairs,
ridges – on the surface that are somewhere
around a billionth of a metre across.
We have known of iridescent structures in
nature for a long time. Robert Hooke identified
them in peacock feathers in the 1600s. Since
then we have discovered that iridescence is
responsible for the hues of many other living
things. That includes insects, like jewel beetles,
and parts of plants, like the marble berry,
which looks just like a deep blue marble.
There is even a mole with an iridescent coat.18/25 December 2021 | New Scientist | 75>