Pigments don't survive fossilization; even though we have fossil skin from dinosaurs, we don't know what color they were. But fossilization does preserve structure. Specimens from a rare cache of 50-million-year-old beetle fossils still show the microscopic layers that produced structural colors in the living creatures, and we can deduce the colors from an understanding of thin-film interference. One fossil showed 80 nm plates of fossilized chitin (modern samples have index of refraction n = 1.56) embedded in fossilized tissue (for which we can assume n = 1.33). What is the longest wavelength for which there is constructive interference for reflections from opposite sides of the chitin layers?