Quote:
Originally Posted by Hornblower
For all we know, a single visible-light photon coming out of otherwise total darkness may not be enough to stimulate even a rod cell. A barely visible star puts some thousands of photons per second into our pupils. I don't know how many rods are involved in a sharply focused image.
If a single cone is stimulated, it tells us only that the light that stimulated it is somewhere in the visible range. All three types respond over most of the visible range, with a large overlap even between the blue and red ones. If I am not mistaken, our sensation of color is the result of the brain's comparison of the relative amounts of output from at least two of the cone types.
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Hornblower, that is generally correct.
IIUC, the way in which rhodopsin (the visual pigment in rod cells) works, it takes the absorption of one photon in the right part of the cell to trigger a nerve impulse. There are two effects here that make it very difficult (or perhaps impossible) to actually
perceive a single photon.
One is that not all photons entering a rod cell get absorbed by retinal (which is the vitamin-A derivative that binds to rhodopsin and allows the absorption of a photon to trigger a conformational change and hence trigger the signalling cascade that leads to the nerve impulse).
The second is that one's visual perception contains a certain background noise. Even on the darkest night, or underground, you never perceive unremitting blackness. You see a bit of noise in it. This noise makes it hard to see very very faint light sources. And it makes the absorption of a single photon pass unnoticed.