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Could an animal have eyes like a reflecting telescope, rather than with a lens? The back of this eyeball is a paraboloid mirror, and the retina is a small body on its focal plane. Because the retina must be small, such an eye would have poorer resolution than a vertebrate eye of similar size.

Are there any organic mirrors in the real world? How smooth is the reflective layer behind a cat’s retina?

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Yes. Scallops have almost exactly this kind of eye:

Scallops Have Eyes, and Each One Builds a Beautiful Living Mirror

Scallops have 200 eyes, which function like a telescope: study

"Almost", because they actually have two separate retinas, an upper retina and a lower retina, with the back-of-the-eye mirror shaped to separately focus the central and peripheral visual fields on different retinae. The mirrors are made of specially structured guanine crystals, with layer spacing optimized for blue-green light (which dominates the scallop's environment).

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  • $\begingroup$ Nature never ceases to amaze me. I assume this structure exists due to low light levels and not necessarily to see things a long way away. But either way... cool juju! $\endgroup$ Oct 3, 2021 at 22:33
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(note: I have to handwave the biological material able to reflect, I'm not an expert on that)

Optic principle remains the same

In fact, this is not very different from the optics we currently have. Our eye lens does the job of focussing now. As far as I know in species with our type of eyesight, the eye lens focuses the incoming light on a particular small region on the retina (called macula), with the highest concentration of receptor cells. Behind this spot, bundles of neuron connections lead the signals to the brain. Using reflection, macula will have to move to the front.

An advanced eye using reflection

A reflective construct is conceivable, see Logan's answer. But Mollusca don't require much, this single eye will require some changes.

Suppose light could reflect from the back of the retina, and land in a macula on the opposite side somewhere..

Below is a sketch, exact geometry would be quite different.. the ellipse of the reflecting back retina would be more elongated (eye made deeper) and point back, reflecting up, near a tilted parabole..

enter image description here

Design around the optic nerve must be changed

I see a physiological issue with the new location of the macula. There is little room for thick bundles of nerves to connect there.. Behind our eyes, large bundles with connections are now kept safely inside.. With nerve bundles going back inward, near the nose and forehead, the optic nerve would become far more vulnerable, that is not in a protected location.. neuronal connections would need a longer bundle, or brain physiology should be adjusted to the new outlet location.

Leaving the eye lens in could give interesting options..

The mammal eye is basically a spheroid shape, filled with fluid. When the retina is reflective and we leave the eye lens out, some kind of transparent fleece would still be needed, to keep the vitreous body isolated from the cornea and iris. Now suppose you would simply leave the eye lens in.. and the retina would be able to reflect effectively.. you will have a 3 lens system, that is cornea, eye lens and retina curvature. This system could be designed with certain muscles, allowing for e.g. microscopic eyesight.

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    $\begingroup$ "With nerve bundles going back inward near the nose and forehead, they would become more vulnerable" - move the eyes backwards. It does sacrifice field of view and stereoscopic field of view though. $\endgroup$ Oct 2, 2021 at 22:36
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    $\begingroup$ Indeed, +1. The eyes need to be more embedded/deeper, or the brain should have a forward extension, protected by a ridge above the eyes. Neanderthal people had a protective ridge like that. $\endgroup$
    – Goodies
    Oct 2, 2021 at 22:38
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The main problem with a totally reflective design is that it has a open body, which is unwanted for a biological surface like the retina, both in terms of protection of the surface and stability of the medium through which the light travels.

Reflective material per se are possible, as demonstrated by the tapetum lucidum

The tapetum lucidum (/təˈpiːtəm luˈsiːdəm/; from Latin tapetum lūcidum 'bright tapestry, coverlet'; pl. tapeta lucida) is a layer of tissue in the eye of many vertebrates. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light available to the photoreceptors (although slightly blurring the image). The tapetum lucidum contributes to the superior night vision of some animals.

If you are going to put a lid of some sort to have a closed body design, it makes sense that it has also optical properties: it has to be transparent, so it makes more sense to have it diffract the light.

In that sense it would be an hybrid design, refractive-reflective.

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    $\begingroup$ Hybrid reflector/refractor designs are called catadioptrics. $\endgroup$
    – Rafael
    Oct 3, 2021 at 13:02

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