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About two posts back (“What Am I” – a short play) I mentioned the question “Does the evidence of our eyes tell us about the true nature of the world?” as one that philosophers have been mulling over for more than 2500 years. I think the question itself might require a bit of explanation.

Let’s say you’re looking at an orange (there’s one right here for your convenience). I know that it’s perfectly natural to believe that your eyes aren’t lying to you about the fact that its colour is, well, orange. It feels as though the evidence of our eyes is enough for us to say something completely non-subjective about the orange – that it is, in fact orange.

An orange.

But that’s not quite true. What is non-subjective (or at least a little more so) is that the orange is reflecting light mainly in the 635-590 nm wavelength range. Your brain perceives that light as being orange in colour, but that doesn’t necessarily mean that “orange-ness” is in any way a fundamental property of that light.

Consider this: visual information is not carried from the orange in front of you to your brain through fibre-optic cables in your head; it’s converted to electrical charges in your eye and then sent along nerves (in a process called phototransduction; and by the way, isn’t it astounding that we actually know this stuff? Three cheers for medical science!) to the brain, where it’s processed to form visual imagery.

In other words, not a single ray of light has ever reached your brain (well, unless you’ve had a lobotomy, but let’s ignore that possibility for now). Light is something that we cannot experience directly. It’s as though we’ll never be able to listen to an opera, but we can make some sense of its sheet music. And more than that, it isn’t even true that the notation in our sheet music is somehow the “right” one and that there’s no other way to record the opera. Many animals see colour very differently from how we do; some can even see ultraviolet light, which we can’t.

As a matter of fact, colour as a whole is an illusion that our brains create in order to help us deal with the world around us. Look at the table below, and consider the “border” between orange and red light – the wavelength of 635 nm. Truth is, it doesn’t really exist. It’s not as though light with a wavelength of 634 nm has a property of orange-ness whereas as we cross over to 636 nm, the light acquires a property of red-ness. That’s just how our brains perceive things, for some evolutionary reason.

This is how human beings perceive light of adequate intensity between 450 and 700 nm in wavelength

Finally, here’s a treat for having stuck around for all this: a black-and-white photograph of a rainbow. Notice that there’s no way to tell that there are different colours in there – black and white cameras don’t have brains like ours, which lump different wavelengths into fixed categories of colour. A rainbow spans a continuous spectrum of colours; the distinct bands that we normally see are an artefact of human colour vision, and no banding of any type is seen in this black-and-white photo (only a smooth gradation of intensity to a maximum, then fading to a minimum at the other side of the arc).

A black-and-white image of a rainbow

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