Part of the show Science of Sight, Eye Diseases and Animal Vision
Kerry in Canada asked:
We all know that when we open our eyes under water we see blurry images. I believe that this is because water has a different index of refraction in air, where our eyes have been designed to work properly. Is it possible for someone to have such poor eyesight in air that they could see clearly under water?
The problem is that in air, light is focussed by both the cornea at the front of the eye and the lens within the eye, and actually most of the focusing is done by the cornea. The reason the cornea can focus is because the refractive index of air and water are different. But as soon as you put your head under water, the refractive index of the water and the liquid within the eye are the same and the cornea no longer focuses the light. The cornea is then lost as a refractive surface. In answer to the question, our eye is said to have a focusing power of 50 diopters. 45 diopters is due to the cornea. So about three quarters of the eye's focusing power is due to the cornea. When you go under the water you lose the cornea as a focusing surface. So you would have to be 45 diopters myopic, which means normally that you would be focussed at just over two centimetres. I've never met anyone who's minus 45 diopters.