As a swimmer objects appear blurred and out of focus. However when the swimmer wears goggles, that keep the water away from the eyes, the object appear sharp and in focus. Why do goggles improve the image formed?
When the swimmer is out of the water, light from objects first enters the eye through an air/cornea interface. Since the air (n = 1.00) and the material of the cornea (n = 1.38) have different indices of refraction, light rays are bent at that air/cornea interface. And since the swimmer - indeed all of us spend most of our time surrounded by air - our eyes have developed to accomodate that refraction. When the swimmer is underwater and not wearing goggles, water (rather than air) is now in contact with the cornea ofthe eye. Since water (n = 1.33) has a different refractive index compared with air, the amount of refraction at a water/cornea interface is much smaller than that at an air/cornea interface. Our eyes simply cannot adapt to such a change because the lenses in each eye are unable to accomodate the different amounts of refraction. As a result a focussed image cannot be formed on the retina and so objects appear blurred.
When the swimmer wears goggles underwater, refraction at an air/cornea interface is restored. As a result the swimmer can see clearly, once again.