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We all know and adore pure beauty of mallard male:

enter image description here

I know that such iridescent colours are usually a result of feathers forming a photonic crystal with layers of air sandwiched between layers of feathers. However as I know (e.g. from observing a layer of gasoline spilled on water), wavelength of the reflected light depends on the angle of the incidence: enter image description here

(The lightwave reflected is a result of constructive interference of waves reflected by each interface. If the angle changes there are different optical paths for reflected waves and different wavelength will be stronger).

So why is duck's head only green and dark green/black? Why aren't there more colours to see, depending on the angle?

user46147
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1 Answers1

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The color of the feathers' structural elements indeed depends on angle of incidence/scattering (and also on position in the feather). But the feather itself is made of a lot of very small elements. We see the whole system as an average of the varying colors of the elements, which are oriented differently.

See e.g. ref. 1 figure 2 (the bars here correspond to $100\,\mathrm{\mu m}$):

Ref 1 Fig 2a,b

These images (a) and (b) correspond, respectively, to asterisk and arrowhead in figure 1d (the bar here corresponds to $2\,\mathrm{cm}$):

Ref 1 Fig 1d

References

  1. Stavenga DG, van der Kooi CJ, Wilts BD. 2017 Structural coloured feathers of mallards act by simple multilayer photonics. J. R. Soc. Interface 14: 20170407. http://dx.doi.org/10.1098/rsif.2017.0407
Ruslan
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