Re: Why do goggles help you see underwater?

Greetings:
Our eyes are wonderful senses which have been optimized for our normal 
living environment. When we look into a body of water our vision into the 
water is often obscured by the surface ripples and by the light from the 
sky being reflected from the surface into our eyes. Glass bottom boats get 
around these problems by having flat glass plates for us to look through 
eliminating ripples and the sky light is blocked off by a canopy or a roof 
above the viewers, giving us a clear, reflection free view into the water. 
The same arguments hold true for face masks or goggles. The flat glass or 
plastic eliminates the ripples and the sky light is blocked by our head at 
the surface and there is no background sky light when we are under water. 
Of course these same arguments hold true for aquariums and they are often 
darkened or the viewing glass is tilted to eliminate reflected background 
light from spoiling our view. We also have observed how a curved fish bowl 
distorts the viewing.

When we wear a face mask in the water it withstands the pressure of the 
water and our eyes experience a near normal environment; however, without a 
face mask the water is directly in contact with our eye and this destroys 
the operation of the optics in our vision system.

WHY? When a light ray passes from air into the cornea of our eye the ray 
bends at the air/cornea surface. This bending is called REFRACTION and the 
amount of bending is determined both by the angle that the cornea surface 
makes relative to the incoming light ray and also by the index of 
refraction of the material (Refraction is caused by a material such as 
glass apparently slowing the speed of light. This effect can be graphically 
demonstrated. Perhaps this is a topic for another question to the Mad 
Scientist). In a material with an index of refraction of 2, light travels 
at 1/2 the speed of light in a vacuum. Light travels at 1/3 the speed in a 
vacuum in a material with an index of 3 etc. Air is similar to a vacuum and 
the index is near 1. Most common window glasses have an index near 1.5. 

You might also want to refer to my answer about lenses in the Mad Science 
archives:

Physics : RE: Why does an image turns upside down in a mag. glass? 
Adrian Popa, Staff Optical/Microwave Physics, Wed Nov 20 11:39:48 1996

Also; before the following discussion  you might want to refer to a Web 
site about the anatomy of the eye at: 
http://www.planitia.net/eyedoc/anatomy.html

As light enters the eye it passes sequentially from air with an optical 
index of 1, through the Cornea with an index of 1.377, the Aqueous Humor 
with an index of 1.337, the Lens with an index of 1.411 and finally the 
Vitreous Humor with an index of 1.338. 
 
The surface of the cornea is precisely curved so that each light ray 
entering the cornea, parallel to the lens
axis, is refracted causing the rays to converge into the lens 
for imaging onto the retina. The indices of the humors inside the eye are 
similar to the cornea and cause little reflection or refraction within the 
eye. The index of the lens is greater that the indices of humors so that 
refraction occurs at both  lens/humor interfaces to form an image on the 
retina. What a marvelous system! 

However; if the cornea of the eye is in contact with water rather than air, 
the key refraction from the first surface of the cornea is eliminated 
because the index of water is near 1.33 closely matching the 1.377 index of 
the cornea. Only a very slight refraction occurs at the surface and it is 
at the wrong angle to enter the lens. This in turn destroys the operation 
of the rest of the eye's magnificent optical system!

       
Best regards, your Mad Scientist
Adrian Popa  with the expert help and guidance from
John Link, Admin, MadSci Network