The reason that one see this effect is the same reason a spoon looks bent when one 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 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 optimised 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.
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 = (n(1) — n(2))/R
Where n(1) and n(2) 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 dioptres.
A dioptre is the unit of refractive power, equal to the reciprocal of the focal length (in metres) of a given lens.
Assuming a value of 0.008 metres for R, the power in air is about 47 dioptres and in water is about 6 dioptres.
The focusing ability of the eye is variable to a degree because the shape of the lens can be controlled by the ciliary muscles.
Published in The Hindu on Jan 24, 2002.