Why is it so hard to see through ocean water when above the surface, but far easier to see when submerged in it?
In: Physics
Light is reflected off the surface so you see a broken image of what is above the water.
Under water, you are not getting that reflection.
If you look up when under the surface, you can see a silvery reflection of the surface too.
Much like sound, light has a hard time changing medium. When you’re trying to see something that’s underwater, from above the surface you’re attempting to see light that’s passed through the air, through the surface of the water, a distance into the water, bouncing off the subject, back through that same distance of water, through the surface of the water into the air, and back to your eye.
The light has changed medium twice in that journey, each time experiencing different angles of refraction and reflection.
When you’re already underwater light behaves similarly as it does when viewing things in the air while in the air. All the light bouncing off the subject and back to your eye remains in the same medium and doesn’t get reflected/refracted as it passes through the barrier between them.
Think of the water like a two-way mirror, some of the light that hits the surface is reflected back, and the rest passes through, but at a different angle, and this happens again to some degree as it returns back through the “surface” of the air to your eye.
It’s purely because the ocean surface is rough. Light is bent when it passes between air and water (because it moves slower in water) so if the surface is rough, images are scrambled. All you need is a thing like a bucket (Americans: think pail) with the bottom made of flat glass. Put that on the top of the water so the glass holds the surface flat and you can see through without trouble. People seeing clearly underwater will have a mask or goggles with a similar, flat, transparent surface.
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