Underwater Magnification

[blacknet]

Hello,

WRONG WRONG WRONG WRONG WRONG!

beyond 39 feet this 2/3's the distance and 33% larger does not hold water. Under 39 feet this rule applies; however it is NOT true that the virtual image is perceived at that range (2/3's and 33%)

Subsequent studies by Kent (1966), Luria et al (1967), and Luria and Kinney (1970) were in agreement that distance estimates for objects closer than one meter were, in fact, too small, but beyond one meter the perceived distances were too large!! Moreover, the distance overestimates increase both with object distance and water turbidity.

further research:
S. M. Luria, J. A. S. Kinney, and S. Weissman,'Estimates of Size and Distance Underwater', Amer. J. Psychol. 80, 282 (1967).
S. M. Luria and Jo Ann S. Kinney,'Underwater Vision', Science, 167:14, 1454 (1970).
H. E. Ross, S. S. Franklin, G. Weltman, and P. Lennie, 'Adaptation of Divers to Size Distortion Under Water', Br. J. Psychol. (1970), 61, pp. 365-373.

Ed

 

[DORSETBOY]

DiverBuoy, have to disagree I have experienced this diving in turbid rivers back home in Wales.

 

[richhagelin]

One way to demonstrate the "bending" of the light is to place an object (coin or whatevere) in the bottom of an opaque glass (like a coffee cup). Position your head so that the coin just "disappears" below the near edge of the cup. Then fill the cup with water while keeping your eye in the same position. As the cup fills with water, the coin will "appear".

Rich Hagelin

 

[DA Aquamaster]

The tendency to over estimate distance in turbid water makes perfect sense. There is more to distance estimation than binocular vision and the angles your eyes use to focus on the object, there are also the other visual cues your eyes use. Haze (or murky water) for example creates the impression of distance and the intervening terrain between you or the object can make it appear closer or farther away.

I have spent a lot of time learning to estimate distances accurately for shooting purposes (The avilability of inexpensive laser range finders have made this skill about as valuable as celestial navigation is when you have a far more accurate handheld GPS in your pocket). But I do see the same sorts of optical illusions occuring under water.

 

[Jai Bar]

The answer is not straightforward. From optics point of view, it depends on the distance between the object to mask's faceplate, and the distance from faceplate to retine. If these two distances are very small one gets a magnification of "1", whereas getting objects farther away and/or increasing faceplate distance the magnification will tend towards "1.33" (number coming from the ratio between water/air refraction indexes). So we are speaking of magnifications between 1-1.33, depending on two factors. For those interested, here is the result of geometrical optics calculation:

M = m*[1+(d+R)/D] / [1+[m*(d+R)/D]]

Where m=1.33 (refraction index water/air), d is the distance between mask and eye, D is the object's distance from mask, and R is the radius eye. M is the magnification, of course. I have plotted the result for various distances in the attached image.


However, our brain is a little more sofisticated than simple optics and while everyone knows and sees a spoon half immersed in water bigger, when looking underwater at objects fully immersed the brain is not so easily fooled by optics we perceive the image in different ways. Many studies have shown that the brain makes adaptation to underwater environment.

 

[The Chairman]

I don't think the figures were an attempt at accuracy. The precise amount of enlargement is not as important as the concept! DON'T TRUST YOUR SENSES. The same holds for the speed of sound. We are not trying to enable them to measure things under water, but rather to gain a perspective on the phenomenon that they will encounter. If it makes you feel better, then things look considerably larger and closer under water, unless you encounter the reverse effect of turgid waters. BTW, I have had that happen to me too, and it will freak you out.

 

[NAM001]

I'm short-sighted and have to wear glasses but I don't wear any (contacts, prescription etc) underwater.

My vision problem doesn't seem too bad underwater compared to above. Could that be due to Underwater Magnification?

sebastian

i am far sighted. i can not read a compass closer than 18" on land. in the water i can read it fine. i use a 2.5 or 3.0 reading glasses. the water gives me the same or near same effect as the glassses do out of the water. i am told that water is equivilant to 3.0 reading glasses.

 

[sbrinner]

For water, the value is (I believe, doing this from memory) 1.33 . (Coincedentally, diamond also has the same IOR; if you put a diamond underwater, it will "disappear". A trick occasionally used in crime movies/novels). For glass it has yet another value (depending on the type of glass).

Actually the IOR of diamond is 2.4 (Well 2.4175 to be exact).

See here for a large list of other IOR's

 

[gj62]

PADI says that in turbid water or water with high particulate, objects can in fact appear further away. My guess is that I don't think it was intended to mean further away than they would appear in air ... but only that they might NOT be AS magnified as they normally are.

Well, you'll have to poke fun at the Navy too - from Chapter 2 - Underwater Physics:

2-6.2 Turbidity of Water. Water turbidity can also profoundly influence underwater vision and distance perception. The more turbid the water, the shorter the distance at which the reversal from underestimation to overestimation occurs. For example, in highly turbid water, the distance of objects at 3 or 4 feet may be overestimated;in moderately turbid water, the change might occur at 20 to 25 feet and in very clear water, objects as far away as 50 to 70 feet might appear closer than they actually are. Generally speaking, the closer the object, the more it will appear to be too close, and the more turbid the water, the greater the tendency to see it as too far away.

 

[Texass]

So those Bud cans really aren't the size of a Fosters? I guess I can stop searching the beer stores now.