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J. Perelman
"Entertaining physics". Book 2.
Chapter 9. Reflection and refraction of light. Vision


Imagine that you have been given the opportunity to stay underwater indefinitely and that you keep your eyes open. Could you there to see?

It would seem that once the water is clear, nothing should prevent to see underwater as well as in the air. Recall, however, about the blindness of the "invisible man", which is not able to see because the refractive indices his eyes and air are the same. Under water we are about in the same terms, as "invisible" in the air. Refer to the figures, it will become clearer. The refractive index of water is 1.34. But the indices of refraction of the transparent media of the human eye:

  • The cornea and the vitreous body is 1.34
  • Lens - 1,43
  • Aqueous humor is 1.34.

You can see that the refractive ability of the eye lens just 0.1 stronger than water in other parts of our eyes it is the same with prelomlyaetsya water. Therefore, under the water the focus of the rays is obtained in the human eye behind the retina; therefore, on the retina the image should appear vaguely, to distinguish anything is possible only with difficulty. Only very short-sighted people can see under water more or less normal.

If you want to imagine, how to draw things under water, wear goggles with strongly scattering (biconcave) glass; then the focus of the rays refracted in the eye, will be removed far beyond the retina, and the surroundings will appear before you in the waist, foggy images.

If people under water to help your vision, using highly refractive glasses?

Ordinary glass used for glasses, there are few suitable: the refractive index of ordinary glass is 1.5, i.e., only slightly greater than that of water (1,34); these glasses will break under water very poorly. Need a glass of a special sort, which is characterized by extremely strong refractive power (the so-called "heavy flinches has a refractive index almost equal to two). With these points we could more or less to see clearly under water (on special goggles for diving see below).

Now I understand why the fish eye lens is extremely convex shape; it is spherical, and the index of refraction is the biggest of all, what we know in the eyes of the animals. Without this, the eyes would be almost useless fish, doomed to life in a highly refractive transparent environment.

Cut through the fish's eye.
The lens has a spherical shape and does not change in accommodation.
Instead of changing the shape changes the position of the lens in the eye, as shown by the dotted lines.

Entertaining physics J. Perelman


System Orphus


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