i was thinking the other day about how my eyes are unable to focus properly underwater, like in a pool. it seemed to me that this may be due to the fact that light may not be refracted properly since a lens is constructed based on a certain angle of refraction to create a focal point. this angle is due to the passing of light at an angle between mediums of differing density (air --> water) if i remember my physics correctly. now land dwelling creatures would have eyes built for air, and water creatures would have eyes made for water. how do animals such as frogs, seals, and penguins see clearly above and below water? do they just have stronger muscles to shape the lens of the eye?
I thought the nictating membrane that amphibious animals have took care of this, acting like swim-goggles. Not sure though.
The membrane can’t function like swim goggles - goggles only work because they let a layer of air be strongly curved against your eye and flat against the water. You need that concave air lens.
Maybe amphibians do this by having flat fronts to their eyes, and having much stronger “crystalline lens” elements in their eyes than we do. We get most of our lensing from the strongly curved cornea in the front, which is fine if our environment always has the same index of refraction. But if an animal had flat eye surfaces, it wouldn’t care what the index of refraction of its environment was.
Or maybe they don’t see sharply. Human vision is very unusually good in the animal kingdom at providing sharp vision over a wide range of focal distances. They say hawks have fantastically sharp vision, but it’s only that good at spotting movement at a great distance. I bet humans are the only animals with vision good enough both to sew and to drive.
In different ways, but the most common is to compensate is with a very soft lens that can be readily deformed by muscles. In addition contrary to Napier’s assessment, a transparent nictating membrane apparently does serve to augment the lens for a variety of diving birds, including ducks.
There can be other adaptations as well. For example bullfrogs, that typically float with their eyes partially above water, have a strong concentration of the aquatic-adapted purple-photosensitive pigment porphyropsin in the upper third of the retina, which processes images from below the water, while the lower part of the retina, with the terrestrial-adapted red-photosensitive pigment rhodopsin, counter-intuitively processes the image from above the water.
Penguins however do not adapt well to being on land - their vision is very aquatic-adapted and they are severely near-sighted on land. Not a big problem - they don’t need good distance vision on land, given the paucity of terrestrial predators to harass them.
- Tamerlane
[somewhat off-topic]
On the contrary, hawks have fantastically good vision which exceeds our own all around. Their retinas pack more cones (as well as having two fovea per eye - one for forward vision, and one for lateral vision, resulting in their peripheral vision being nearly as sharp as their forward vision), giving them a higher all-around resolution than humans. Outside the fovea, their vision resolution is roughly twice that of humans, while inside the fovea, it can be as much as 8 times higher than a human’s. Indeed, raptors in general have perhaps the best eyesight of any animal.
Further, they are capable of switching focus between near and distant objects much faster than we mere humans (the ability to switch focus like this is called accomodation, and birds and reptiles in general tend to be much better at it than mammals) - a necessary trait when pursuing fast-moving bunnies at high speeds, or in finding bunnies from on high in the first place.
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very informative and interesting. thanks all!