Underwater Magnification

[jplacson]

Here's the thing, usually all your mind needs is a point of reference to do the necessary optical correction. Even in air, if no point of comparison is present, we can't gauge size or distance properly.

Only problem underwater is that it's not our turf... our brain still can't memorize the small nuances that we've learned over the course of our childhood when it comes to estimating size and distance... stretching your hands out in front of you will give you a better chance of sizing things up... and having a dive buddy beside or near and object helps determine size and distance as well.

 

[don3334]

Here is why you are confused.

The PADI training books say both.

In the PADI Open water diver manual - product no. 79180 version 2.2 (rev(10/01)) i found

Page 75/76 - "When light changes speed going from water to air (like when it enters your mask), it shifts its course slightly (this is called refraction) which magnifies every-thing about 25 percent."



In the PADI Encyclopedia of Recreational diving Third edition Product number 70034 (rev 2/08) version 3.02

Page 4-15 - "You can say, then, that an object appears to be 25 percent closer that it actually is or 33 percent larger that it actually is."


In the Padi Diving Knowledge Workbook product number 70214 (rev 9/05) version 2.0

Page 1-16 Objective 1.2 answer key 3 "When viewed underwater, objects tend to be magnified by a factor of about 33 percent "

Even the books can't get it right.

the correct answer is:

Underwater, refraction at the plane port bends light rays so that the object appears to be 25% closer to the port than measured. Assuming that the separation of port and lens is small compared with the object distance, the effective underwater magnification is therefore F/(0.75*O - F). With the object at a distance O in air, we need to know what focal length F' produces this magnification. Clearly, it must satisfy the equation

F'/(O - F') = F/(0.75*O - F)

Solving for F' we get

F' = 1.33*F

The effective underwater focal length (or magification) is 33% greater than in air..

 

[Andy077]

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

It's even more complicated than that! The speed of light through a medium is dependent on both the composition of the medium AND the wavelength of the light. That's how the prism effect works, white light entering a new medium (water, glass, diamond, etc..) is slowed down, and due to Snells law, bent, but each wavelength is slowed down differently, resulting in the splitting of the white into it's components colors.
Now to add another layer of complexity, observations of gamma ray bursts in distant galaxies are suggesting that the very high energy gammas (shortest wavelength) are just a hair slower than lower energy (longer wavelength) photons, arriving a few minutes later after a journey of 500 Million light years.

 

[SteveAD]

As I tell my students, Don't sweat the numbers. 25%, 33% whatever. Objects will generally appear a bit larger and closer under water. Then I tell them the "Lobster Story"

While diving along the wall at Folley Cove I saw a HUGE lobster right at the limit of visiblity. As I swam closer I realized that he was BIG, but not a monster. Closer still and I was pretty sure that it was a keeper. And when I actually put a hand on him it turns out to be a short.

Percieved magnification is a function of water turbidity, distance to the object, light level, lack of comparative reference, and a handful of other factors. Does the exact number really matter?

 

[ScubaSteve]

Now that was an impressive thread revival. I am surprised you could find the pulse . This thread was a good read....thanks to all. However my philosophy when relating the IOR of water and scuba diving to non divers is "The shark was THIS BIG!!!"

 

[Walter]

It's even more complicated than that!

He said that over 5 years ago....

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