Sonke Johnsen opens his book The Optics of Life by noting that vision “is about light, so perhaps we should start with what light is.” And then, with admirable candor: “I have no idea.” Though it surrounds us almost constantly, light’s true nature is not intuitive. Physicists contend that it exists both as an electromagnetic wave and as particles of energy known as photons. The specifics of this dual nature needn’t concern us. What matters is that neither guise is something living things should obviously be able to detect. From a biological perspective, perhaps the most wondrous thing about
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