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Underwater Magnification

Discussion in 'Marine Science and Physiology' started by scubagirl15, Dec 18, 2003.

  1. scubagirl15

    scubagirl15 Instructor, Scuba

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    I have been confounded by the following conflicting information about magnification underwater:
    - objects appear 25% larger
    - magnification is 4/3
    - objects appear 33% closer.

    What's the real story? Do objects appear 25% or 33% larger underwater? Do objects appear 25% or 33% closer underwater? Is it some other number? Have been trying to figure out the math, but no can do for this puny mind.

    Help!!

    Thanks,
    :)
     
  2. jrtonkin

    jrtonkin Nassau Grouper

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    I think you've switched your numbers around...

    Objects appear 33% larger (which is 4/3 magnification), or 25% closer.

    To see how the math works, consider an object that's 3 meters tall, and 4m away from you.

    If it looked 33% larger, it would look 4m (33% of 3m is 1m, added to the 3m of its true height) tall. So you'd be looking up
    at the top of it at an angle of 45degrees. I.e. a 4m tall object at
    a distance of 4m.

    If it looked 25% closer, it would appear 3m tall, at a distance of 3m (25% of 4m is 1m, so move closer by 1m). This gives the same angular size; I.e. 3m tall at a distance of 3m.

    Hope that helps,
    Jamie
     
  3. cudachaser

    cudachaser Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Cocoa Beach, FL
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    Think about this way, the 8 ft shark you are looking at is really only 6 ft long
     
  4. crispos

    crispos Instructor, Scuba

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    Good explanation. How is it that some books say the effect is caused by light travelling at a different speed in the water. I thought the speed of light was a constant (186,000 miles per second). Or was that speed in a vacuum only?
     
  5. Big-t-2538

    Big-t-2538 Divemaster

    # of Dives: 100 - 199
    Location: Dayton, OH
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    That is precisely the reason things look diferent uw than they do on the surface..this is essentially how a telescope or magnifying glass works....it alters the speed of light travelling in certain directions by the shape of the lens.
     
  6. cudachaser

    cudachaser Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Cocoa Beach, FL
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    Light's velocity slows whenever it passes through any medium, air, water, glass etc. It will slightly bend...that's how lens work

    Joe
     
  7. Sebastian

    Sebastian Solo Diver

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    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?
     
  8. crispos

    crispos Instructor, Scuba

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    Sorry, I had a rough day....so the speed of light is NOT a constant?
     
  9. jrtonkin

    jrtonkin Nassau Grouper

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    Nope, the speed of light isin't a constant. It varies with the material being travelled through. (Ok, it's a bit more complicated than that, but for our purposes, we'll treat it as though it's just a property of the material)

    The commonly quoted speed of 186000 miles/sec is the speed in a vacuum.

    When light crosses from a material where it has one speed to a material with another speed, it bends. The amount of bending is characterized by a value called the "index of refraction".

    For vacuum, this value is 1, by definition. For air, its close enough that you can generally call it 1. 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).
    The exact amount of bending depends on the values of the IOR for the two materials (the one being left, and the one being entered). The closer the two values are, the less
    bending will take place.

    The amount of bending that occurs is also affected by the angle at which the light hits the surface. Look up "Snell's Law" to get the full math. (it's only basic trigonometry, nothing too scary).

    So lenses work by controlling both the angle the light hits the surface of the lens at (i.e. the lens' shape), and the Index of Refraction of the material the lens is made of.

    As for the short-sited question...

    Imagine parallell rays of light travelling through the air.
    When they enter your eye, they bend towards each other slightly, and with correct vision, will meet each other at the same point that they hit the back of your eye (the retina). I.e. the image is in-focus at the point where you can sense it.

    If you're short-sited, your eye bends the light too much, so rays that are parallell in the air cross before they reach
    the back of your eye. But for things that are close, the light coming from them isin't quite parallell, but is diverging slightly. So when your eye bends it, they don't get bent quite as far and you're able to form a focussed image on your retina.

    Now move underwater. The water has a larger IOR than air, and is closer to the IOR that the lens of your eye has. So the light gets bent less in general. So the rays from a distant object now get focussed further back in your eyeball, and if your lucky, might even meet each other at your retina, giving the effect of correct vision.

    For lots more about this, check in the chapters on Optics in most physics textbooks.

    Jamie
     
    Sebastian likes this.
  10. DiverBuoy

    DiverBuoy Instructor, Scuba

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    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.

    The poking fun part is that I've spoken to physicists, professors, and dozens of industry professionals and divers about this and none has ever - not once - not ever observed this phenomenon. One Course Director told me that in a back room over coffee one day a very high ranking PADI long-time official admitted that this fact was made up - but then just became part of the curriculum. Needless to say there have never been any test questions on this specific item.
     

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