Any one who has common sense will remember that the bewilderments of the eyes are of two kinds, and arise from two causes, either from coming out of the light or from going into the light, which is true of the mind's eye, quite as much as of the bodily eye ...
– Plato, The Republic
Rhodopsin hurts.
In your eyes, rods and cones embedded in your retinas detect photons. Electromagnetic waves falling into the wavelength-spectrum of visible light pass through the space between earth and sun, deflecting in certain frequencies off everyday objects — things like gemstones, trees, and dresses. The photons strike your cornea, are inverted by your lens, and are projected as an image onto your retina.
We are creatures of chemicals. We move to the flow of mathematics, of atoms trading pieces. Sometimes the result is fire, sometimes light.
Under the right circumstances, the best part of going to the movies is waiting at the theater entrance for your eyes to adjust to the dark.
"Can you see yet?"
"No," she laughs.
You are getting used to her laugh. But you are not getting used to the smell of her shampoo on your pillows, or the way light seems to find its way to her even in the dark of a midnight bedroom. You are becoming more aware that, while technically you need matter to bend light, this growing affliction of yours is doing the job nicely enough.
For once, the feeling of being a fish out of water makes you thankful.
The in-between blindness you experience when moving between light and dark is due to levels of rhodopsin on the retina.
Rhodopsin is a marriage of two compounds — opsin protein scotopsin and cofactor retinaldehyde. Its misnomer, "visual purple" originates from its enthusiastic absorption of green-blue light. If you have a color wheel handy, you see that, minus the green and blue gradients, you are left with red and violet. Rhodopsin is purple.
Light is an agent of chemistry. When you shine light on rhodopsin, the scotopsin and retinaldehyde become unstable, bleach, and separate, triggering a complex series of chemical reactions that ultimately changes the electric charge within the rod. Thanks to jennifer for tackling that. Now, a basic knowledge of neuroscience becomes necessary. Altering the electrical charge in a neuron causes said neuron to either increase or decrease its firing potential. In rhodopsin, the change in charge triggers inhibition. So, paradoxically, the neurons in your rods fire less when you encounter bright light.
When you are in bright light, rhodopsin levels deplete. So, when you leave a very bright place, you are frequently unable to see even in a place that's only fairly dim. Your rhodopsin is all split apart and frazzled — it's sensitive. Likewise, staying in a dark place for a long time causes rhodopsin levels to build, until you walk out into the sun and are blinded by your overabundance of light receptors. These, respectively, are called dark adaptation and light adaptation — really, your response to fluxuating levels of rhodopsin.
The flash of pain you experience when turning on a light in the morning is the mass breakage of scotopsin and retinaldehyde. Electricity, light, neurotransmitters. We are creatures of chemistry.
Rhodopsin hurts.
One day after adolescence you realize that, for years, you have been taller than your father.
He has resorted to eating healthy. He eats brocolli for calcium, fiber for digestion. He remarks that he's been eating carrots like crazy, but that hasn't helped his night vision.
You remember when he told you, a muddy school-ager, that carrots help you see in the dark — an attempt to breach the lifelong embargo you have levied against all things vegetative. You remember that, once, his fingers were nearly the length of your torso.
His remark about night vision is a joke, and he smiles, because he remembers too. There are too many lines in his face.
Awareness hurts.
The body can't make rhodopsin. It synthesizes rhodopsin from β-carotene. You find β-carotene in, well, carrots.
Rhodopsin is not a hardy substance. Once split, the rhodopsin in any given rod can take half an hour to regenerate. β-carotene undergoes cleavage in the body to become Vitamin A — or, retinol. Paradoxically, retinol becomes retinaldehyde through light-bleaching. Retinaldehyde becomes rhodopsin by combination with scotopsin.
The resulting structure is promptly destroyed by racing photons. Repeat process.
Naturally, higher levels of β-carotene provide more fodder for the constant bombardment of light. And, naturally, in the absence of light, it causes rhodopsin levels to build more quickly, shortening the interval required for dark adaptation.
eien_meru adds that the Stephanus pagination for the Republic quote above is 518a.
Major General Panic informs me that rhodopsin found its way into popular literature — in The Demolished Man, guards are disabled with a device that temporarily banjaxes rhodopsin.
Sources
Pastorino, Ellen & Doyle-Portillo, Susann. What is Psychology? Thomson/Wadsworth: Belmont. 2006.
Wikipedia
http://en.wikipedia.org/wiki/Rhodopsin
World of Colour
http://www.chm.bris.ac.uk/webprojects2003/rogers/998/Rhoeye.htm
Republic quote reproduced here from Flowers for Algernon: Keyes, Daniel. Bantam: New York.
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