The reason that we can’t see clearly underwater is the same reason a spoon looks bent when you immerse it in a glass of water.
Light travels more slowly through water than it does through air.
When light moves from one medium to another, it changes speed, and as a result the beam is bent, or refracted. The amount of bending depends on the ratio of the speed of light through each medium.
The human eye is delicately balanced to make sure that light coming in through the pupil is focused onto the retina at the back of the eye.
But it’s optimized for light coming from air and hitting the surface of the eye. The eye has evolved to take account of the refraction that takes place at the interface between air and eye, and gives a focused image on the retina.
However, when light comes directly from water to the eye, the light is bent by a different amount, so the light isn’t correctly focused. Goggles restore the air/eye interface and normal sight is resumed.
This phenomenon of light bending when it goes through different media is used to our advantage in spectacles, where lenses bend the light to correct imperfect vision.
The amount of refraction or bending of the light depends on the difference of the refractive indices of the media on the two sides of the surface of the cornea, the bigger the difference, the more the light is bent.
Because the refractive indices of air, water, and the cornea are 1, 1.33, and 1.38 respectively, this difference is much smaller when the eye is in contact with water than when it is in contact with air.
The power P of a surface is given by P = (ni n2)/R, where n and n2 are the refractive indices of the cornea and the medium outside it respectively and R is the radius of curvature of the cornea. P is measured in diopters. A diopter is the unit of refractive power, equal to the reciprocal of the focal length (in meters) of a given lens.
Assuming a value of 0.008 meter for R, the power in air is about 47 diopters and the power in water is about 6 diopters.
The focusing ability of the eye is variable to a degree because the shape of the lens can be controlled by the ciliary muscles.
However, the increase of power which this can produce is much less than the loss of power of 41 diopters when the eye is in contact with water. In fact, the biggest change the eye can achieve is about 15 diopters in young children, dropping to about only 1 diopter in 60-year-old adults.
This means that the eye in water is unable to bend the light entering it enough to focus it onto the retina, so objects appear blurred.