It's still based on quantum principles. That means only photons of a relatively narrow range of energy values (seen more below) will be absorbed that would cause the proper excitation of the electron, which is the trigger that ultimately leads to the brain perceiving the stimulus (after a very complicated process;
@evan-e-cent gave a more detailed description in post #67). Any more energy or less energy in the photon, and the excitation does not happen.
It's the photon energy that is critical here. (And the photon energy is directly proportional to its frequency, so if you had to pick between frequency and wavelength here, frequency would be the one to choose, given its proportionality to energy).
That's not correct. It is certainly not what I meant convey anyway.
“Red” photoreceptor cones are not particularly sensitive to blue light even though blue light has a higher photon energy. And that is completely consistent with the underlying quantum nature at its core. Just because a photon's energy is significantly greater than the allowed change of electron energy states it does
not mean that the electron will necessarily absorb some or all of that energy. Instead, quantum theory predicts that that the electron state will likely not be impacted by the photon at all.
(Light frequencies anywhere from near-infrared all the way through ultraviolet will be absorbed by the eye in one way or another. It’s just that a particular color cone type is only sensitive to photon energies within a limited range. And that is not inconsistent with quantum theory.)
Let me illustrate a more simple example.
Consider a light source with a uniform spectrum of light (with no gaps in the spectrum, and let’s assume the spectrum spans at least the visible band, if not the infrared and ultraviolet too). Shine that light through a cold gas (any gas will work, but hydrogen or neon might make good choices), then observe the resulting spectrum. You'll notice that now there are very narrow gaps (called spectral lines) in the resulting spectrum (after passing through the gas)! Photon energies higher than a given spectral line passed right through the gas as did photons of lower energies. Only certain energies are absorbed.*
*(In this simple gas case, the spectral lines are not infinitesimally thin, but do contain a very small bandwidth, which can be explained by the Doppler effect of the moving gas molecules. It's way more complex for [non-gaseous] huge molecules working together as a tissue.)
The molecular structures of human photoreceptors are far, far more complex than a simple gas (not to mention not being in a gaseous state). There’s more complex mechanisms for absorption and thusly the absorption bandwidths are larger than the simple gas example. Yet extraordinarily more complex as they may be, they still follow the same quantum rules.
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