Transparent eyeballs?
July 28, 2013 7:01 AM   Subscribe

If the human eye were completely transparent, what would be the effect on our light perception, colour perception, and ability to focus? Are there any species with transparent eyes? (I'm asking as background for an SF story).
posted by Ballad of Peckham Rye to Science & Nature (17 answers total) 1 user marked this as a favorite
 
The retina has to be opaque to visible light in order to function, so I imagine that such an eye would be able to see colors/wavelengths that we cannot see.
posted by brownrd at 7:06 AM on July 28, 2013


what would be the effect on our light perception, colour perception, and ability to focus

You wouldn't be able to see because the way that you see is by light being absorbed by your retinas.
posted by Tanizaki at 7:06 AM on July 28, 2013 [1 favorite]


Eyes are light detectors. They work by focussing and absorbing light.

A truly transparent eye is one that lets all light through it without focusing, polarising or absorbing any of it. Not much use as an eye, really.

However, back-of-an-envelope idea: you could possibly build an *astonishingly huge* eye that lets almost all the light through it. Add a bit of quantum zeno effect handwaving and you could have a planet sitting in a stellar cluster that is the retina of a galaxy-scale eye that is lensing kilometer-scale photons oh please stop me now....

So, no.

If it lets light pass, it's no use as an optical detector. You can't have an invisible eye.
posted by Combat Wombat at 7:17 AM on July 28, 2013 [1 favorite]


Yeah, if a material is transparent to your, Ballad of Peckham Rye's naked eye, you can't make a retina out of it that can 'see' the kind of light that you, BPR, can see. That said, light is just one part of the electromagnetic spectrum - radio and UV are two other examples, and if you have a material (such as glass) that's transparent to (lets through) the visible spectrum but blocks some other part of the spectrum, I guess you could build a retina that 'sees' that.

So, to take a hypothetically case, (some) glass blocks (some) UV radiation. I'm neither a biologist nor a physicist, but if I was reading an SF story, I wouldn't boggle at a creature whose eyes were made of glass and was 1) blind to visible light but 2) saw UV.
posted by heyforfour at 7:23 AM on July 28, 2013


Response by poster: Just to clarify, I realise you wouldn't have functional vision. But there are different forms of blindness, and I want to know how it would be experienced.
posted by Ballad of Peckham Rye at 7:23 AM on July 28, 2013


Have a SLR camera handy? Take off the lens and take a picture.
posted by munchingzombie at 7:25 AM on July 28, 2013 [1 favorite]


But there are different forms of blindness, and I want to know how it would be experienced.

This would be the "total" form of blindness. It would be as if you did not have eyes at all. You would see what you see behind your head.

If you need to have a character who is invisible by some means, I think that you can get away with having them with normal vision. People are used to invisible characters and it is not going to be immersion-breaking for most people.
posted by Tanizaki at 7:50 AM on July 28, 2013


If the retina is there, but can no longer react to light, because the light is passing through it instead of being absorbed by it, I imagine it may be perceived as a Closed Eye Hallucination.
posted by radwolf76 at 8:02 AM on July 28, 2013


It could be some kind of compound eye, like flies have, encased in a transparent ball.
posted by gjc at 8:35 AM on July 28, 2013


It could be transparent to light, but opaque to some other wavelength. So it would be transparent to a normal human, but the eye would see radiowaves, microwaves, etc.
posted by spaltavian at 8:41 AM on July 28, 2013


Best answer: As the other posters have said, the eyeball needs to have visible components to function.

Fundamentally, an eye needs a pigment which can capture the photon. Called a chromophore, this is a specific electronic transition in a molecule which will absorb a photon. This absorption causes a change which can then be registered by the cell, and the nerves connected to the cell as a signal: "Light!" In the visible spectrum, chromophores are dark in the colour they absorb. Light goes in, but doesn't reflect out. Chromophores frequently reflect photons of colours which are not quite of the same wavelength. This gives us visible reflective colour.

Secondly, to be an eye that gives a focussed image rather than just a spot, most biological eyes have lenses. Lenses don't absorb light, they redirect or scatter it. Lenses therefore can be transparent, indeed better ones are more transparent than poor quality lenses. They will however be visible from the distortion effect they produce, just as a raindrop is visible. In large numbers, clear lenses look like, are, clouds or the white caps of breaking waves. A single lens, however could be very hard to see. Lens distortions come in many types: rainbows and geometric distortions are common, and typically quite visible.

Finally, most eyes need some way of moderating the light they can take in. In most biological eyes, this is the iris, mimiced by the aperature in a camera. The problem with chromophores is that they can easily get saturated by too much light, and get stuck in the "on" state. Often called "bleaching", this is why bright lights blind you for a while, and what photographers are complaining about when they call something burnt in. A functional eye needs light control.

So: to function as a light detector at all, an eye needs some chemical which can absorb the light it wants to see. That, by definition, is visible on that wavelength. Secondly, most eyes need lenses to make resolvable images. Lenses can be transparent, but are visible due to distortion effects. Finally, most eyes have light controls, to avoid saturation problems. These have to be opaque, and thus reflective, to the frequency of light being blocked.

One trick: you can make the eye function in a non-visible wavelength. An eye that functioned in the terrahertz spectrum might be very hard to see with a human eye.
posted by bonehead at 8:46 AM on July 28, 2013 [3 favorites]


If everything but the retina was transparent, it'd be like the SLR with no lens example above: completely out of focus. You'd only be able to see how bright and the average color of the scene you're looking at.
posted by zsazsa at 9:17 AM on July 28, 2013


As this is for sci-fi here's some pseudo-scientific conjecture:

If you change what you mean by transparent you might be able to get some wriggle room. Normally we use it to mean light passes straight through something without interacting so it's impossible to extract information from that light. However, if you just want the eye to be invisible from all directions that might be possible to achieve. As a heuristic; imagine a cube of TV screens, with tiny cameras in between the pixels projecting the images onto opposite screens (like the James Bond invisible car) and you sort of have a transparent cube. A more organic way of achieving this could be due to quantum entanglement, as a photon passes through the 'eye' it becomes entangled with another photon. The new photon is then absorbed whilst the original photon travels out of the back of the eye, as if nothing has ever happened.

Assuming this process works for photons entering the 'eye' at any angle, it would be invisible to any external observer. You would also be able to see in all directions simultaneously which sounds cool, though it gives me a bit of headache trying to visualise what that must be like.
posted by Ned G at 10:34 AM on July 28, 2013


How about a partially-transparent eye? We need the back surface to absorb light so that we can actually detect/see anything, and we need the front lens to focus the light (so, like glass, the tissue is transparent (clear, non-absorptive) but has a sizeable index of refraction to bend the light, and an observe sees reflections off the front of it)
Pupils look black because there's no light coming out and not much reflecting off, but really that whole column from the front lens to the back retina ("column" but it's more cone-shaped than cylindical) is clear, filled with transparent optical fluids (no real tissue there, to the best of my understanding).
Aside from that, what else is in the eye? Maybe there's not an essential reason that our irises are colored, or our whites are white. You can imagine a functional eye in which those tissues are clear. The iris includes muscles that control the size of the pupil and the amount of light that gets though the imaging system; for fiction purposes, maybe you can have clear/transparent muscles... or maybe those muscles aren't there, and there's much more potential for damage from bright lights and almost no night-vision. Having the whites of the eyes clear would be a problem, would mean not blocking non-focused light (not the pupil) from the detector at the retina, so the image would have this huge unfocused bright background, like watching a low-quality projector-type movie outside in bright sunlight.
You could imagine sheathing the imaging column in something opaque, and then the rest of the eye could be transparent, or absent, but it would be more like "let's imagine a non-spherical eye" than "let's imagine a totally transparent eye".
posted by aimedwander at 11:59 AM on July 28, 2013


It would be a kind of "all-white" type of blindness, if that's what you're asking.
posted by rhizome at 12:02 PM on July 28, 2013


Best answer: Hey again, I realised I missed the point of the question earlier, sorry about all the physics wankery.

To try to answer it properly: if the retina were transparent, I guess it would be total blackness as in effect there isn't any light incident on it.

If the retina is functioning as usual but everything else is transparent I imagine over-exposure is the issue, as the iris wouldn't be able to control the amount of light entering the pupil, so as others have said 'all-white' blindness. It might even be blindingly bright, as there would be way too large an aperture for normal tolerances. Also, as the white of the eye would be transparent, you'd have the issue of direct sunlight shining onto your retina if you go outside. I'm not sure if being in the sun would blind you or not without a lens focussing the light, but if so going out in the day would be really dangerous.

In low light levels the transparent eye would pick up more light than a normal eye, but as most of it isn't being focussed the result would be incredibly blurry. You'd probably see the average colour of your surroundings as a single colour covering the entirety of your visual field. The lens would still be functioning (I think) so it's possible that you would see what a normal person would superimposed onto the incredibly blurred background, though it would seem very washed out as there wouldn't be any areas of darkness visible. If you were looking at a single (dim) light source in a dark place (with no scattering of walls etc) I suppose vision would be as normal.
posted by Ned G at 2:54 PM on July 28, 2013


This technology might be akin to something that biology might eventually evolve. Instead of evolving an iris and single lens, it might evolve rods and cones with higher dynamic range that doesn't need an iris. Combined with the microlens thing and some different neural processing, it could be a fantastic visual system.
posted by gjc at 7:07 PM on July 28, 2013


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