A commercial for Dawn dish soap shows an image of a cute baby seal, its eyes like large black marbles. Random images lead to random thoughts; in this case, seeing underwater without a mask.
Light slows down when it goes from a less-dense medium to a more-dense medium. Humans’ eyes are adapted to function when light goes from the atmosphere (in practice, a vacuum) through a lens, and through the vitreous humour such that the light is bent to the focal point. I assume that the vitreous humour is similar in fish and mammals; and anyway, why complicate things for a simple question? Let’s assume that we’re only dealing with the lens.
In air, light we see goes from ‘fast’ to ‘slow’ as it goes through the lens, which bends it toward the ‘normal’ (a line perpendicular to the surface). In water, the light doesn’t slow as much so it isn’t bent as much. Since water is less dense than air, light going from water to the eye doesn’t get bent as much going through the lens, right?
So could a human see normally under water if he wore a converging contact lens?
Yes, a human could see things in focus with a correcting lens to compensate for absence of the strong focusing of the air/cornea surface, which is replaced with a water/cornea interface underwater. It is hard to make a correcting lens because of the relatively high index of water. One approach would be to make a concave lens out of air (encapsulated in plastic, for example), either in the form of contact lenses or glasses. This would eliminate the fogging and pressure of goggles, but would likely result in lots of distortion, unless a more complicated multi-element lens were used. If you make some, I would like to buy a pair, or at least try them out.
There have been designs for underwater glasses, but they’ve been of the over-the-ears type. You don’t need diverging lenses – you need converging lenses, because the bending at the water-cornea interface is much less than it is at the air-cornea interface. This means, perversely, that the lenses must actually be thicker at the edges and thinner at the middle if you’re using, say, lenses with air inside. (Curiously, I haven’t seen lenses with higher-than-water index material used)
I don’t know what you’d make the lendses out of. The hydrogel they make soft contact lenses out of has an index between that of water and of glass. I think for this you’d want a higher index. You’d probably do better with hard contact lenses. You might be able to find a sufficiently high plastic, but I suspect you’d be forced to use glass.
No, I’m talking about glasses that you hang on your ear, your eye is still in water. The glasses could be made with two concave pieces of plastic, with air between them, with the air gap thinnest in the middle. As CalMeacham explained, the reason the the two surfaces are concave for a converging lens is that the index of the lens is less that that of the water on either side of it.
Well, I have worn soft contacts for 35 years and am now doing monovision contacts (one for short distance and one for long. I have worn my contacts underwater without a mask for many years with no distortion whatsoever. They don’t wash out of your eyes when you are submerged.
I will add that it is not recommended to wear contacts underwater without a mask due to possible loss of lens or bacterial infection. However I am offering a data point that I have done it many times and see fine underwater.
What I’m talking about is a lens that would correct the refraction of light in water, such that a person’s vision would be the same underwater as it would be in air without the lenses; not wearing contacts underwater.
I’m not actually looking for such lenses, or suggesting that ones be made. This is just a thought experiment about the bending of light.
BTW: If anyone can draw an illustration and post a link to it, it would help me understand how it can be done (if it can be done). Basically, I’d like to see parallel light rays going into this lens, being bent by it, proceeding to the eye’s lens, and then converging on the focal point of the eye. Exact angles and indexes of refraction are not important; just a conceptual illustration.