When it enters the eyes, is light a particle or wave? If so, what happens to this particle or wave when it is ‘seen’? Is it absorbed physically?
In: Biology
Yes. The photons are physically absorbed by electrons in the molecules that form the (photoreceptor) cells in the retina.
The energy transmitted by those photons change the physical properties of the receptors and they release a neurotransmitter that eventually signals your brain that light hit there.
When the photons reach the retina in the back of your eye, some of them penetrate through to the photoreceptors. These photoreceptors have a protein called rhodopsin that is embedded in the membrane, rhodopsin has a molecule inside it called 11 cis-retinal which when a photon hits it, it supplies just the right amount of energy to convert it to another isomer or another shape called 11 trans-retinal. It’s a very simple conversion, but this now allows the molecule to slip out of rhodopsin and go to the inside of the cells (photoreceptors) and activate a protein called transducin, which breaks into multiple parts, one of which goes and activates another protein called phosphodiesterase which starts to hydrolyze or break a molecule called cGMP. When cGMP becomes low in the cell, a cation channel (which means a membrane channel that allows positive charge through) closes, no longer allowing calcium and sodium ions into the cell. Now the photoreceptor is more negative inside and this negative or “hyperpolarization” wave spreads to the cell’s other end. In the absence of light, photoreceptors are constantly secreting a neurotransmitter (signal chemical) called glutamate, when the negative wave reaches the release site after there is light, glutamate stops being secreted. Then this affects the downstream cells (there many many cell circuits in the retina, and it’s quite complicated to explain) which causes a signal to be sent to your brain. You have many of these photoreceptors and their spatial organization is conserved in the representing neurons that reach your brain (visual cortex). There are also many circuits that for example inhibit the signal from nearby circuits in the retina, this allows for the sharpening of vision. And then your brain processes all this and fills in gaps from patterns and memory to produce the image you “see.”
I am not a physicist, so I don’t know about the wave/particle state of the photon as it activates retinal. But I hope I helped you understand the biology of photon to signal.
The best video animation of the whole vision process I ever saw is at https://youtu.be/AuLR0kzfwBU
Edit: English and elaboration
Light is both a particle and a wave. It depends on what phenomena you are observing. All what happens in the eye can be explained by wave theory. Thus, refracted at the lens, imaged on the retina, detected by rods and cones cells.
As it always is with light: it’s complicated.
Light interacts with itself as a wave, which is actually why blue eyes exist (the light destructively interferes with itself in the iris)
When the light is absorbed by the retina, it can hit one of 4 (or more for some people) cells. Rod cells (which sense light level and nothing more) or the 3 (sometimes 4 or more) types of cone cells which are sensitive to specific bands of the spectrum. The light interacts with these cells as a particle. When the light is absorbed, it causes the cell to release a signal, causing you to see a single point of light
The light itself is converted to a negligible amount of heat energy, unless it is a focused or incredibly intense amount (looking at a laser or the sun). Then not only are the cells damaged by being overloaded with light, but the retina can be physically burned.
The photon is a wave. All fundamental particles are waves. Do not listen to people who tell you there is a wave-particle duality.
Waves of what? Waves of probability distributions.
Wtf does that mean?
It means that what is more fundamental about reality is that particles are this “wavy” thing, and when you look at it, it will appear as a particle with large probability where the wave is large, and low probability where the wave is low.
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