Fish tank distortion

[Squirrel]

I can just about understand why there might be refraction of light when your head is at an angle to the tank, but what about when you look from the front, straight at the back? It still looks like the tank is not as wide as it really is, when I must be looking right along the 'Normal line.' Can someone explain this to me?

[lisaj]

I'm guessing it's because when you focus on something, your eyes are not pointing straight ahead, but are rather converging at an angle. This angle is amplified by the change in refractive index.

[Squirrel]

That's an excellent theory, but I just tried looking with one eye closed and it still looks just as thin. I'm beginning to think it's just my eyes, because several people I've asked (who, admittedly,don't actually own or live with a fish tank) swear it looks normal width to them!

[wildwood]

I can't explain it, but it is very interesting when you think about it. It is the same as when you look in straight from the top, the depth of the tank doesn't look as deep as what it is.

I do remember reading something about it having to do with our eyes and the different properties between water and air. We can not focus in water and air at the same time, this may explain why, if you were to stick your head in the water your depth of field would be correct.(not suggesting this though:-)

[lisaj]

ok, even with one eye, you've got a lens (your cornea) focussing light that comes in from slightly different angles. These angles would also be wrong (since our brain is focussing based upon air), and are also affected by the change in refractive index. Just another theory, but possible also.

[FTVS]

Could this be a simple as the magnifying affect of water? I know from diving that evrything appears nearer than it is and objcets appear 25% bigger - could it simply be that thw back of the tank looks 25% nearer? Or Or am I being a bit Friday afternoonish here

[FTVS]

clearly Friday afternoon typing anyway!

[Squirrel]

After lots of scribbles and diagrams, I think I can see what you're getting at, lisaj. When the light rays come out of the water at an angle, they are bent away from the normal line, so when they cross over and enter the eye, the rays make a larger angle with each other so the image of the back of the fish tank is bigger. I hope I understood that correctly.