If you ask "why are leaves green?" the usual answer is "because they're full of chlorophyll, and chlorophyll is green," even though "why does chlorophyll scatter green light?" is a great question too.

"Rayleigh Scattering", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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If you ask "why are leaves green?" the usual answer is "because they're full of chlorophyll, and chlorophyll is green," even though "why does chlorophyll scatter green light?" is a great question too.