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Humans cannot see clearly under water without goggles. How do aquatic mammals solve this problem?

• For light reflecting off an object to be perceived as anything more than dim diffuse illumination, it must be focused on a single point on the light-sensitive retina at the back of the eye.

The divergent light rays that strike the front of the eye must therefore be bent (refracted) to varying degrees in order to form an image. Light is refracted when its waves cross at a glancing angle from one medium to another with a different refractive index.

In terrestrial vertebrates, light is refracted mainly by the curved surface of the cornea, whose refractive index is considerably higher than that of air. The eye’s lens has a similar refractive index to that of the surrounding parts of the eye, and is responsible only for around one-third of the refractive power of the human eye, serving mainly to adjust the fine focus of the image seen.

Underwater, the cornea becomes ineffective as its refractive index is very close to that of water.

The underwater world becomes very blurry because light is focused a long way behind the retina, and we become, in effect, very long-sighted. This can be rectified by putting air back in front of the cornea with a face mask or a pair of swimming goggles.

The same obviously cannot be true for animals that live underwater because otherwise their eyes would be of little use.

Animals such as fish, cephalopods and aquatic mammals overcome the loss of a refractive cornea underwater by possessing more powerful, spherical lenses that can deal with this problem, unlike the lens in the human eye. Next time you eat a fish, take out the lens and you will see it is shaped like a marble.

“Next time you eat a fish, take out the lens and you will see it is shaped just like a marble”

The real question is how some animals, such as diving birds, see clearly in both air and water.

Ron Douglas, Saffron Walden, Essex, UK

This article appeared in print under the headline “Optic aquatic”