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In 1634, Sir Theodore de Mayerne, physician to
Charles I,
observed that the 'eyes' on the wings of
the peacock butterfly "shine curiously like stars, and
do cast about them sparks
of the colour of the
Rainbow; by these marks is it so known that it
would be needless to describe the rest of the body
though
painted with a variety of colours."
| Peacock spreading its feathers. |
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The iridesent plumage of the peacock originates in
the
fine side branches of the feathers. Every branch
carries a series of bright colored spots, the actual
colour depending on the exact position of the
spot on
the branch and on the angle of the incident light.
| Detail of a peacock feather. |
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In general,
the diversity of bird feather colours can
be explained by a mere two factors: pigments on one
side and simple structures
in the feathers which
interfer with the incident light. Pigment particles are
embedded into the newly grown feathers
during the
moulting season. They absorb light of certain
wavelengths or disperse the reflected light and thus
contribute
to the colour of the plumage.
What is a peacock feather up close?
Each feather consists of thousands of flat branches
(as above in the
detail). When light shines on the feather,
we see thousands of glimmering coloured spots,
each caused by minuscule
bowl shaped indentations.
Stronger magnification reveals microscopic lamellae
(thin platelike layers) at
the bottom of the indentations
(see photo below). Just like in butterfly wings, the
regular pattern
of the lamellas leads to interference
phenomena and thus to iridescent colours. This
principle is also responsible for
varied colours of
pheasants, birds of paradise and humming birds.
| Down side of the feather. |
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Each branch has round indentations, with
micro - lamellas
on the bottom of each indentation
which disperse the incident light, colouring the feathers.
| Close up of feather |
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| overview |
Is it really just interference?
Yes. For example, the white colour of this albino
peacock is
due to the missing black melanine
pigment. The usual rich colours of the peacock are
seen because black pigment which
absorbs most of
the incident light, allowing us to see only the
interference colours. In this peacock, the interference
is
still happening, but the effect is entirely washed out
by the abundance of white light. In this albino, you
can see
that the "eyes" of the tail feathers are clear,
not coloured.
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