Iridescence
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
The iridescence of the Blue Morpho butterfly wings.
Iridescence is an optical phenomenon characterized as the property of surfaces in which hue changes according to the angle from which the surface is viewed, such as soap bubbles and butterfly wings.
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IridescentAmmonite.jpg
IridescentAmmonite.jpg
A fossil ammonite displaying iridescence.
Iridescence is caused by multiple reflections from multi-layered, semi-transparent surfaces where the subsequent phase shift and interference of the reflections modulates the incident light (by amplifying or attenuating different frequencies).
The word is derived in part from the Greek word ιρις (transliterated as iris, meaning stem, whose plural is ιριδες, transliterated as irides). In Greek mythology, Iris is the personification of the rainbow, who acted as a messenger of the gods.
See also thin-film optics.
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Colours_reflected_from_a_thin_water_film_depending_on_thickness_and_angle_of_incidence.png
Colours_reflected_from_a_thin_water_film_depending_on_thickness_and_angle_of_incidence.png
This computed image shows the colours reflected by a thin film of water illuminated by unpolarized white light. The radius is proportional to the thickness of the film, and the polar angle is the angle of incidence.