Will changes in water density affect perceived object magnification?

[Santa]

Ok - let me see if i can get my point across - physics are not my greatest forte.

We all know that objects appear magnified by about one third as light is refracted passing from the denser medium of water - to air.

The "about" aspect in basic discussions has been explained by the fact that it plays a part how far the lense is from your eye - and how close or distant the observed object is.

But: The colder water is, and the higher its salinity the denser it should logically be. Increased water density affects the angle of refraction. I believe that is described as the refraction index for at certain liquid at a certain temperature.

Now my question is - Will changes in water density and consequently the refraction index as light passes through the mask lense affect the degree or scale of diver-perceived magnification of an observed object? If so one might imagine things to seem further magnified in very dense liquids than the normal factor of one third.

It was a trip to Egypt that started me thinking about all this. To me most of the regular reef fish somehow seemed bigger there than in most other places I'd been. Like they were all on steroids. Then I got this idea that the particularly high salinity of the red sea might play a part. I've scoured the web for information about this but found nothing directly applicable to diving.

What do you think? Any diving optometrists out there?

[sambolino44]

No. It will affect magnification, but not enough to perceive, unless you are way more perceptive than most people.

[Santa]

Speedy reply thanks!!! This has been haunting me and I was beginning to even get into the slowing of light particles as they travel through more athmosphere in the evening as well ;o)

Apparently then - Either the fish in the red sea really are on steroids or by some strange process I am without knowing it.

Sorry bout your cam mate - I guess its natures way of saying "get an even bigger and better one!". Well ... nature prompts but rarely funds. That was the very same way nature invited me to get a D6 instead of my old mosquito. Nature is kind of the big, burly room mate from hell that way.

[altaskier]

The index of refraction scales with density. Freshwater is (if I remember correctly) only about 2-3% more dense than seawater, and the density variations with temperature are much smaller. I don't think you can measure absolute distances to 2% accuracy by eyeball, which means you can't judge magnification to that accuracy by eyeball.

[Santa]

Seawater is denser than freshwater due to the salt content. Fresh water has a density of 1000 kg/cubic meter vs. an average density of 1027 kg/cubic meter for ocean salt water. Which fits your estimate anyway so in essence what youre sayin makes sense.

[altaskier]

Seawater is denser than freshwater.

Oops... you're right.

[ed.hamilton1]

High water density is more likely to cause visual reversal where objects appear further away than they actually are as if an object appears hazy are brain assumes it is further away however the more you dive the brain adapts and compensates for this and the object is where it appears to be.

[Santa]

I think my brain is habituated about now ;0)

Based on personal experience, I'm not so sure I agree that increased salinity has similar effects as turbidity, seeing as the salt is dissolved. Egypt and Dubai are probably the saltiest waters I've dived, and also among the ones with the best viz.

The turbidity factor, as I interpret it is mainly the optical illusion of increased haziness and colour desaturation working on the brain which is more of a physiological and psychological issue than a physics related one, it seems to me. If I'm overlooking something here with respect to refraction please let me know.

On a personal note I don't feel much affected by turbidity either anymore in terms of judging distance or depth, but certainly that is adaptation and to a slighter degree deliberately weighing the probable influence of various sets of contributing factors. I recently had the chance to revisit some sites that I must have dived within my first 15-20 dives years and years ago. I very clearly remember my then depth calls on how far below the sand bottom was - and my guesses were like twice the actual depth ;o) (probably partly due to all the intensity and excitement of learning to dive).

I've made a habit of making educated guesses before checking any gauge or other dive parameter - because I may need those quick judgment calls when I'm with students or guiding - and these days I'm rarely off by much.