GETTING FOCUSED UNDERWATER - SOME LIGHT READING

If you have spent any time in the water and tried to look around while submerged, you’ll know that nothing is in sharp focus.  Even if the water isn’t making your eyes sting or otherwise irritating them, objects you try to focus on just will just not resolve into a crisp image.  If you are above the surface and peer down through transparent water, objects below are in focus, so what happens to that clear vision when you dive below and try to get a closer look from within the water? And why, if you put on a mask or look porthole or the side of an aquarium, are underwater objects in focus?
 
IT’S JUST BASIC PHYSICS
The word physics can be intimidating to some of people, I get that.  Fortunately, you aren’t one of those people.  Optics is the branch of physics we need to look at to understand vision and the way light behaves in both air and water.  Light can travel through air, water, the vacuum of space and any other transparent substance.  These media all have a influence on how fast light can travel through them because the tiny packets of light called photons interact with the particles it is traveling through.  Light is fastest in a vacuum because there is no matter to interact with and slow it down.  The symbol scientists use for the speed of light in a vacuum is a c (it is taken from the first letter of the Latin word, celeritas meaning swiftness).  The speed of light is a universal physics constant whose exact value is 299, 792,458 meter per second.
 
BENT LIGHT
When light moves from one medium to another it changes speed.  The refractive index (or Index of Refraction) of a material is a number that defines how fast light travels through some medium.  The refractive index is calculated by dividing the speed of light in vacuum by the speed of light in the material of interest.  All transparent matter has its own, unique refractive index.  Below is a table of the refractive indices of some common substances.  An interesting interactive tutorial was created by Nikon allows you to how changing different variables effect animated refracting rays.

​SHARPER IMAGES
Notice the close the refractive index of air (1.00033) is very close to that of a vacuum (1.00000).  Compare that to the refractive index of water (1.33).  When a light ray enters water its velocity drops by a factor of 0.75188 times (1/1.33).  This change of speed results in the bending of light rays (refraction).  The “bend” you see in a drinking straw where enters the water in a glass is the result of refraction.  The tissues of your eye (cornea, crystalline internal lens and the gelatinous humors) all cause light to be bend and create a focused image on your retina.  Your eye is adapted to deal with light passing through air and into it.  When light passes through denser water with its 1.33 refractive index the lenses in your eyes bend it at a different angle that causes the rays to form a focused image behind your retina. The image behind your retina results in a blurry image.
 
SEEING CLEARLY NOW
The best method to get view well focused images underwater is to get the air back in front of your eyes.  That is where masks come into the picture.  They put a pocket of air between objects you are viewing and your eye; allowing for the formation of sharp, focused images. 
 
WARNING: OBJECTS ARE CLOSER THAN THEY APPEAR
But the mask also exacts a price in terms of image distortion.  Objects will be magnified by 33% due to the additional refraction created by the mask’s glass and air pocket.  These same objects will also appear to be 25% closer to you.  Some other distortions you might notice include pincushion distortion and lateral chromatic aberration are noticeable, although these are usually not notice by recreational divers.
 
THE BOTTOM LINE
When you first start diving the image distortions produced by changes in refraction may cause you to make mistakes when estimating the size of marine life or how far an object is from you.  This is nothing to worry about, as your brain soon learns to compensate for the distortions.