Iridescence in feathers occurs when light waves reflected off certain structural layers within flattened barbules “interfere” with one another.

Within the barbules, each layer of melanin granules is the same thickness. Therefore, for a given incoming light angle, the color that is brightened is the same not only for each layer
individually, but between different layers as well.

The total brightness you see is the sum of all these layers acting together, and the more layers involved, the brighter the color. (Hummingbirds have 7-15 layers.)

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