I used to wonder about this as a kid.
Light travels bazillions of miles across the universe and zaps my eyeball.
What then?
What happens to light after it enters the eye?
How is it transformed, and into what?
And after that?
After traveling trillions of miles, the light is focused by the lens to form an upside-down image on your retina. There it is absorbed by pigments in the rod and cone cells. The pigment molecules are activated by the light and cause an electrochemical impulse which propagates within the retina. Several layers of nerves in your retina begin the processing of the optical signals. The results are propagated by the optic nerve into other sections of the brain.
I have a damaged retina due to (presumed) octular histoplasmosimosis. I have very wild “light shows” when I close my eyes at night for sleep. What causes this?
(sorry for the semi-hijack!)
Sounds a bit like this. I had something similar after a few sleepless nights in hospital, as though my brain was trying to start the dreaming before I was properly asleep. Does the section on “Sensory phenomena” sound familiar?
re: the OP - As above. Note that the chemical changes require an energy input, so some of the light energy goes to this; the remainder is absorbed as heat or reflected and re-emitted from the eyeball (normally too small an amount to notice, tho’ the optician can shine a light into your eyes and see your retina, which he couldn’t do if they didn’t reflect any light).
From this very brief description, there are a number of things it might be. It could be hypnagogic imagery, as suggested by Malacandra, but that has nothing to do with having a damaged retina. Many people with perfectly healthy eyesight experience hypnagogic imagery. Other possibilities that come to mind are some sort of phosphene activity, or Charles Bonnet syndrome. Could you give a bit more detail of what you experience (and perhaps also of how your waking vision is affected by the damage)? How sure are you that the damaged retina and the “light shows” are connected?
To Penguinlady, although what JWT Kottekoe says is true, the nervous activity that light generates in your retina, your optic nerve and your brain is merely triggered by the light. The energy that powers this activity comes, ultimately, from the food you have eaten. The actual amount of energy in the ligh entering your eyes is very small, and in the eye’s receptor cells it turns first into a form of chemical energy (chemical changes in the receptor cells trigger the nervous process) and eventually into (a very small amount of) heat.
When you look at a rose: photons of light strike the photoreceptive cells in your retina and are individually absorbed by either rod or cone opsins, which emit a signal to ‘polarize’ the cell. When millions of these cells become polarized, ganglion cells in the eye automatically send signals back along the optic nerve to the visual cortex of the brain in layers of ‘bars’; certain numbers of bars are detected as edges, corners, motions or colours. Specific neural signals are then forwarded along neural pathways to other parts of the brain according to what is detected, and those other parts of the brain react to the new information accordingly.
Just to note: Light isn’t “transformed”. Photons hit your eye, and your eye just detects them and starts sending signals to the brain. Your body doesn’t go on to use those photons, they just bounce off. It’s the same with touch: if a tennis ball hits your skin, your skin detects it and your nerves start sending neural signals up to the brain. The tennis ball just bounces off; your body doesn’t transform it.
OK, so either my brain is getting fuller and fuller of received photons, or someone looking at my eye should be able to see what I just saw because it’s bouncing off…
No, the don’t “just bounce off”. The ones that you “see” are actually absorbed, and disappear.
Whenever the question is “Where does the energy go?” The answer is almost always “heat.” Your head warms up some after it’s all said and done.
I’m pretty sure it’s true that the photons (or at least a percentage of them) that hit the photoreceptor cells are absorbed, this is what causes the energy change that sets off the chemical reaction in the first place. Just like the tennis ball has less energy when it bounces off: some of it went into stimulating those nerve cells.
Your eye (not the brain) is, in fact, getting “fuller and fuller of received photons”. It’s called “getting heated up”. The heat then is dissipated by the body in various ways.
No the photons are absorbed, and they eventually turn into heat, as I already said (but such a small amount that you will not notice). A few may bounce off, but the retina is not like a mirror.
Neither is correct, really. Some of the photons reflect (bounce), but these are of no interest to us, because your eye does not get whatever information is carried by any photon that happens to reflect off the retina. The only photons that contribute to your vision are ones that are directly absorbed by photosensitive pigments in the rods and cones in the retinal cells. What does absorbed mean? It means the energy of the photon moves an electron inside an atom inside a molecule inside a rod or a cone. The photon now only exists as the quantum mechanical energy level of the electron. Does it stay with that electron forever, “filling up” your retina? No. Your rods and cones have a relaxation time that allows them to be excited by new photons. So what happened to the first one? It could be that the quantum mechanical energy is emitted again as a new photon in a random direction. Much more likely, though, the absorption of the photon into the photopigment changed the shape of the pigment molecule, which allows it to have the biochemical reaction that instigates the nerve response that sends a signal of light into the brain. After that, it probably has yet another biochemical reaction that relaxes the pigment (read: takes away the excess energy) and readies it for a new signal. So where does the energy go? Into another molecule inside your retinal cells. Almost certainly it eventually dissipates as heat.
Getting heated up is not the same thing as getting full of photons. When the photons are absorbed they cease to be photons. Their energy is transformed into another form.
“Their energy is transformed into another form.”
That’s what I was wondering. And if that form is heat, then the eyes of a seeing person should always be a little hotter than the rest of the body? What about the brain?
No, because all of the external bits of our body are constantly absorbing photons. The only difference with the eyeball bits is that they’re set up to signal the brain that they’re doing so.
And what about the brain?
The heat, by the way, is the reason that it’s bad to look directly at the Sun. When you do, a whole bunch of photons end up getting absorbed by a very small portion of the retina, and that small portion of the retina gets burnt.
The part that says “re-emitted…” Could that also be heat? After my stroke (vision damage) a “fog” kept forming in the middle of my right eyeglass lens for months, and that never happened before.
Like all of your body, your eyeball is warm, and so will emit a small amount of heat. I do not believe that a stroke could cause your eye to emit more heat (or moisture) than usual. That will not have been the cause of the ‘fog’ that you experienced. It seems much more likely that the experience of fogginess was actually the result of a problem with the visual processing in your brain, caused by your stroke.
Any exposed skin is also absorbing some of the photons that are incident upon it, and will warm up accordingly. Heck, the exposed surface of your eyeball is also absorbing some portion of the light incident upon it.
As to whether one’s retinas are warmer than other parts of the body? Unlikely in most cases; your retinal tissue and sclera behind it have blood flowing through them, and vitreous humor in front of them, all of which help to carry the received heat away to other parts of the body. In most cases, the heat input is also negligible next to the heat produced by metabolic activity. Moreover, the exposed surface of your eyeball experiences evaporative cooling any time your eyelid is up.
One exception: exceptionally bright light sources, such as lasers and sunlight. In these cases there is a tiny image of the source on one part of your retina that may receive an awful lot of heat - enough to cause thermal damage. This usually requires deliberately staring at the sun for a prolonged period, but in the case of lasers, the damage can happen in a fraction of a second.
Bottom line: despite the heat input due to photon absorption, it’s doubtful that one’s retinas are measurably warmer than any other part of the body.