“Blue,” writes Alexander Theroux in The Primary Colors, “is a mysterious color, hue of illness and nobility, the rarest color in nature. … It is the color of anode plates, royalty at Rome, smoke, distant hills, postmarks, Georgian silver, thin milk, and hardened steel. … ”
Blue, or what to our mind is blue, is a short light wave, about 475 nanometers long (a nanometer being a billionth of a meter). The violet light wave, the shortest we can see, is even shorter than blue. Bluer than blue and shorter than violet is ultraviolet, which we can’t see, and x-rays, which we can’t see either. Both ultraviolet and x-rays have very short wavelengths. Red is the longest light wave we can see (650 or so nanometers long). And even longer are infrared waves, followed by microwaves, followed by radio waves.
Blue colors our moods and our music. A blue mood is a low mood, a mood wherein we might sing the blues. The word blue is low on the vowel scale (upon which sigh is high). But blue also means royal, pure, heaven. The blue I most marvel at is that deep blue-black polish of a clear night sky right before dawn.
Blue, though, is all in our mind.
Color vision is given to us by our six million cones, light receptors that form part of the retina. Cones crowd together into the center of the retina, called the macula. At the center of the macula is a pit called the fovea. The fovea is packed with cones and specialized for visual acuity and color. We are foveal animals: to focus on an object zooming our way, we must turn our foveae toward it.
Cones contain pigments that make them react to particular light waves. Most of our cones are long-wave (red) and medium-wave (green) cones. (A cone flirts more with its given light wave, but it’s not monogamous.)
We have many fewer short-wave (blue) cones. Blue is further foiled by a yellow pigment in the macula that absorbs short light waves. Blue, in our brain’s opinion, muddies the view.
So how do we see blue?
The retina transforms light waves into electrical pulses. The pulses relay through the retina’s layers from cones to retinal ganglion cells. The retinal ganglion cells have long axons that together make up the optic nerve. Electrical pulses travel down the optic nerve to a part of the thalamus called the lateral geniculate nucleus. So far we have seen nothing. These are not sights but rather bits—a billion bits per second—of information.
The lateral geniculate nucleus relays the pulses back to the visual cortex (located under that occipital bump at the back of your head). Now the subjective experience we know as vision begins to happen. Just begins to happen, because there is more, as the visual cortex now projects the pulses forward.
But what about blue?
Besides our six million cones, our retina has 120 million rods. (Rods sit on the periphery and react to dim light.) A mere one to 1.5 million retinal ganglion cells transmit all this and more (brightness, edges) to the lateral geniculate nucleus. The ganglion cells are relaying long waves minus medium waves or long waves plus medium waves—sums and contrasts.
Moments there are when the light is bright but the red cones and green cones are sitting there doing nothing. To our brain, this looks a lot like blue.
