# Underwater Vision

While in contact with air, the normal human eye does a wonderful job of feeding accurate visual information to the brain. In contact with water, however, it fails miserably. To understand why, we need to examine the eye's optical properties. The figure below shows a lateral cross-section of the human eye (taken from Hallett et al). The number accompanying the label of each eye component is its index of refraction. In particular, note that the index of refraction of the cornea, n(c), is 1.38.

The refractive power of the cornea when it is in contact with air (which has index of refraction n(a)=1.00) is

`                  P(a)={n(c)-n(a)}/R`

`                  P(a)=47.5 diopters.`

If, instead, the cornea is in direct contact with water having index of refraction n(w)=1.33, its power is

`                  P(w)={n(c)-n(w)}/R`
`                      = 6.3 diopters`

The cornea has lost 41.2 diopters (87%) of its refracting power! This means that, underwater, the eye becomes severely hyperopic (or hypermetropic), so that parallel rays of light entering the relaxed eye are no longer brought to a focus on the retina, but well behind it (see below) so that everything is a blur.

Can accommodation by the lens of the eye overcome the cornea's loss of power? Unfortunately, no! The best that the ciliary muscles can do with respect to rounding the lens is to make up for about +16 diopters, and that is only for very young children. For adults the maximum accommodation by the lens is approximately +10 diopters at age 30, and this number decreases almost linearly to +1 diopters at age 60