Following on the parent's comment on the human eye : One of the most striking graphs I've seen was the plot of the transparency of water vs optical wavelength.

Following on the parent's comment on the human eye : One of the most striking graphs I've seen was the plot of the transparency of water vs optical wavelength. Basically, there's only a tiny transparency gap, which coincides almost exactly with the wavelengths the human eye is sensitive to. http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/watabs.html http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/watabs.h...