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If somebody were to ask you, "what color is a mirror?" What would you answer? Well, you might instinctively (by dint of representations in popular portraiture and the like) suppose that mirrors are silver-colored. Or you might contend that mirrors are actually clear, or are devoid of color. But in fact, the color of mirrors, counterintuitively, is very close to white. But not quite white, as will be explained shortly. But firstly, why are mirrors closing in on white? Consider the nature of color. Light comes in wavelengths, with different wavelengths being different colors. In a rainbow (like this one), the lens of the millions of water droplets in the sky reflects the suns opposing rays at visible angles (though the rainbow continues for quite a ways in either direction in wavelengths beyond the visible spectrum). The rainbow appears at all because the water droplets are reflecting all different wavelengths of light at different angles. Similarly, an object typically appears to be a certain color because it is reflecting the wavelength of light which is that color -- and absorbing the rest.

But then we come to objects which are white. They are absorbing no color, and reflecting all. So why are not all white objects mirrors? Because their inconstant surfaces are scattering all those wavelengths of light, even as they reflect them. So the secret of the mirror's white is its smoothness, its reflection of not only all wavelengths, but the direction from which they come. But there is no such thing in the human experience as a 'perfect mirror.' There is no true perfection; so the question then arises, in what direction do our imperfections flow? This can be found out, in the case of a mirror, by standing between two mirrors facing one another. As one gazes down the ever-darkening 'tunnel' created by this reflection (ever-darkening because the mirror does absorb some light), an unmistakable greenish tinge inevitably emerges. But why green? For the same reason that most plants are green. Green cracks the spine of the spectrum. It lays in the middle, with the remaining wavelength options spread out evenly in either direction from it.

One might intuitively think that the color on the spectrum closest to white is yellow, being a bright color which contrasts highly against white's nemesis, black. But in fact, the closest visible thing to white is green. And so, a mirror reflects green away just a bit more efficiently than it does for every other color, approaching pure white but falling just short of it due to the imperfections of reality, with a subtly greenish hue compounded many times over when reflecting into itself. And, over time and millions of generations of evolution by natural selection, this is what both plants and sub-basking reptiles like most lizards have discovered, and have to a great degree congregated around. By reflecting green, they are able to absorb the maximum range of other wavelengths of light (without overheating, as they would if they were black).

Now, all of this is especially fascinating in light of the early reports of the background color of our Universe. You might recall that some years ago astrophysicists announced that they had calculated the average of all the wavelengths of color generated across the whole of observable existence. And, though these physicists initially reported that it was indeed a shade of green, they later backpedaled and announced this to result from an error in calculation, the true color being a far less exciting beige. And since our Universe will shift from a bluer average color in its beginning from all those blue-burning young stars, to a reddish hue towards its end, a green average color would have signified a metaphorical (though not chronological) midpoint in our Universe's life cycle.

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For ScienceQuest 2013

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