CoachColors that shift with the viewing angle — soap bubbles, oil slicks, peacock feathers, the back of a CD — are iridescence: no pigment, but interference in white light, reinforcing some wavelengths and canceling others.
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Example 22.18 (Soap films and oil slicks)
Light striking a soap film reflects partly off its front face, partly off its back: two coherent copies whose path difference is set by the thickness and the viewing angle. Each thickness returns some wavelengths reinforced, others canceled: the film reflects a color that slides as it drains and thins. An oil slick on wet asphalt plays the same trick.