Marine Mammal Sensory Systems pp 97-106
The Cornea as an Optical Element in the Cetacean Eye
In the human eye about two-thirds of the total refractive power resides in the cornea. The large difference in the refractive index between air (1.0) and corneal tissue (ca. 1.38) makes the anterior corneal surface the principal refracting element, although the curvatures of the surfaces of the crystalline lens are stronger. This is due to the fact that the lens is immersed in aqueous humor (n = 1.33) which is not very different in refractive index from the surface of the lens (n = 1.36). If a diver submerges in water, the effect of the cornea is almost completely cancelled since the refractive index of water (ca. 1.33) is about the same as the index of the cornea. Humans are therefore extremely hyperopic (farsighted) under water. The refractive power of the lens alone, even if fully accommodated, is insufficient to focus light on the retina. Humans have to wear a mask that reinstates an air-cornea interface in order to see well in water.
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