Despite the oxymoron, we humans can’t really see light. We can only see objects that are illuminated by light, not the light itself. The light-sensitive components of our eyes – the so-called ‘rods and cones’ that respond by to radiation in the ‘visible’ part of the spectrum – don’t ‘see’ the radiation at all (despite what it says on the box). They respond to light reflected off objects. And actually, only the part of spectrum that hasn’t been absorbed by the surface we’re looking at.
Orange and Un-oranges
We see the orange as orange because the un-orange part of the visible spectrum is absorbed by the surface of the fruit. The remaining radiation is reflected away from the surface where the light particles (photons) enter our eye where the real magic begins…
The light radiation, travelling as ‘photons’ or particles-that-behave-like-waves, first arrives at the cornea (the outer part of the eye covered by contact lenses if you happen to wear them); then moves through the pupil (the aperture or hole in the iris which varies in size according to ambient light); is cleverly focused by the lens; moves on through the ridiculously-named jelly-like wasteland of the ‘vitreous humour’; then finally arrives at the retina where the heavy lifting is performed.
The retina contains a stack of cells which respond in their own special way: the thin strips we know as ‘rods’ are by far the most plentiful – humans have a massive matrix of about 120 million cells per eye – which are mostly sensitive to light volume (that’s luminance or brightness in the graphic-arts world); and the 6 million-odd collection of red, green and a relative few blue-sensitive cells which kinda look conical although I, for one, would never have described them as ‘cones’ if it was my job.
If you want to get picky they’re not specifically red-green-blue sensitive at all but we’ll deal with that little issue in another post…
The rods and cones do a remarkable thing: they convert the light particles into electrical signals by a process called ‘phototransduction’ before passing the signal on through the optical nervous system via synapses and connections in the same way that other brain functions are transmitted. There are some double-ended (bi-polar) cells first, then an array of ‘ganglion cells’. There are less of them than the rods and cones (about 100 cones connect to one ganglion) and we didn’t know much about them until the 90s really but now we think they’re important for non-imaging reasons like psychological resonses to colour, circadian rythms etc.
Anyway, after all that is said and done, the relative electrical charges which were sympathetic to the original light source pass on to the brain by way of the venerable optic nerve. After that, it’s a process of neurology which leads to the business of perception and recognition. So if certain rods and cones are stimulated in a particular way then the brain determines it must be orange we’re seeing. Simples.
But is it orange?
We call it ‘orange’ without really knowing if there’s a universal experience of orange, or whether my orange is the same as your orange, or without prejudice as to whether orange is good or bad or indifferent, or whether it makes you feel happy or sad, randy or glad. Etc.
We can chalk this up with other unknowns of human experience like, ‘does a tree make a sound when it falls (without audience) in the forest?’ and ‘does the refrigerator light go out when we close the door’ or even ‘what happened to
Schrödinger’s cat?. But surely there’s a way we could determine wheter the the experience of orange is in some way universal or least common.
Well, not exactly but there is an area of the brain we call the ‘colour centre’ which is critical in the perception and processing of colour signals received by the eye, which ultimately results in colour vision. With this in mind, we can map the stimpulation of the brain in response to the presence of a colour but is it then possible to distinguis the colour from the thing itself. And even if we could, what about the philosphical question as to whether we are seeing the same thing.
We’ll deal with this biggie question in another article. There’s no short answer!