Morpho Wing Structural Blue: 2D Layer Cross-Section
Rays bounce off every chitin/air interface; the reflections add up for one band of colour.
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Experiment: peak colour versus tilt (measured vs predicted)
| Angle | Matrix-method peak (nm) | Bragg prediction (nm) | Difference |
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The science: multilayer interference and its limits
A Morpho wing scale carries ridges with stacked chitin lamellae (n₁ about 1.56) separated by air (n₂ = 1). Reflections from all interfaces are in phase when 2 (n₁ d₁ cosθ₁ + n₂ d₂ cosθ₂) = m λ, with sinθ = nᵢ sinθᵢ. With d₁ = 70 nm and d₂ = 125 nm the first-order peak is near 468 nm. Tilting shortens the optical path, so the colour moves towards violet.
The spectrum is computed with the full transfer-matrix method (mean of s and p polarisation), independently of the formula. The same physics module drives the 3D version, so both give identical numbers. Simplifications: lossless, dispersion-free indices; flat, perfectly periodic layers; one scale; no diffuse scattering. Colours integrate R(λ) with the CIE 1931 colour-matching functions (analytic fit) under flat white light; the melanin base absorbs the transmitted light.