Focusing Under Water?

This question was answered by Professor Ron Douglas...

It is true that amphibious animals, such as ducks, seals and turtles, can see well in both air and water. For humans, however, the world becomes all blurred as soon as we stick our heads under the water. This is because in animals such as ourselves that live in air, two parts of the eye focus light: the lens within the eye, and the cornea, which is a transparent window at the front.

Of these, in humans, the cornea does about three quarters of the focussing because there is a large difference in refractive index between the air and the cornea.

The lens in our eyes is relatively flat, and is mainly responsible for fine focussing of the image, as we look at things at different distances, by slightly changing it's shape, becoming fatter as we look at closer objects.

Our world becomes blurred underwater because water and the cornea have very similar refractive indices, so the cornea no longer focuses light. We therefore become very long sighted under water, as our lens is not optically strong enough to focus the light.

What something like a duck does, therefore, is when it is in air, it has the same basic eyes that we do; with a cornea that focusses most of the light, and a flattish lens. When it goes under water, however, when the cornea no longer focuses light, it pushes its soft lens against a quite hard iris, and part of the lens bulges through the pupil, forming a sort of nipple on the front surface of the lens.

This acts as a very powerful lens, and allows the animal to see underwater, when the cornea isn't working as an optical surface. This allows diving birds, for example, to both successfully hunt for fish underwater, and to catch the bread that you throw for them on the surface.

Interestingly, there is a group of humans that seem to see quite well underwater; these are the Moken, who are wandering sea gypsies inhabiting the coast off Thailand and Malaysia. They make a living by diving in the sea, often without goggles to harvest things like abalone.

It turns out that when you compare their ability to see detail underwater to a similar group of Europeans, the Moken do much better. Any camera enthusiast will tell you that if you want to see a large range of distances in focus, in other words, to have a large depth of field, you close down the aperture of the camera.

So when the Moken go underwater what they have learned to do is to close down their pupil, giving them a large depth of field, and compensating for the long sightedness induced by losing the cornea as an optical surface under water. Interestingly, given time, European children can learn to do this as well.