Colors vs depth & 2 other questions

[MyDiveLog]

1. I've read several texts that say the water filters out the colors of the spectrum from shortest to longest wavelength, in the R.O.Y.G.B.I.V. order. Red goes at about 20', Orange at about 25', Yellow at about 30', Green at about 40'....but Blue, Indigo, and Violet continue past where most folks ever dive. An instructor I know says that all the texts are wrong, that Indigo and Violet disappear at relatively shallow depths. Does anyone know if this is true and, if so, why? The 'depth versus wavelength' story makes intuitive sense, but not his version.

2. Another instructor said that an average human, when they fully expand their lungs, adds about 12 lbs of lift to their body. Does anyone know the volume change in the human body with a deep breath to see if this is true? It makes a great story for beginner divers, to teach them to exhale when trying to begin their descents, but I'd like to actually know if it's true.

3. How far can you hear a boat under water? I've heard people say 1 mile and others say 10 miles ... anyone have any facts?

Thanks

 

[Drew Sailbum]

1. Longest wavelengths (Reds) are lost first. Violet is the short wavelength end of the visual spectrum.

2. I don't know about average, but I can generate a 8lb swing pretty easily.

3. It depends in part on teh size of the boat and how much noise it is making. I have clearly heard a ferry passing about a half mile away.

 

[Xanthro]

1. I've read several texts that say the water filters out the colors of the spectrum from shortest to longest wavelength, in the R.O.Y.G.B.I.V. order. Red goes at about 20', Orange at about 25', Yellow at about 30', Green at about 40'....but Blue, Indigo, and Violet continue past where most folks ever dive. An instructor I know says that all the texts are wrong, that Indigo and Violet disappear at relatively shallow depths. Does anyone know if this is true and, if so, why? The 'depth versus wavelength' story makes intuitive sense, but not his version.

Your instructor friend is incorrect. Shorter wavelength has more energy and penetrates water more deeply.

Technically, the water doesn’t absorb the light, particulate matter in the water absorbs the lights energy. Perfectly clear water would allow light to pass through.

Here's a website that explains it.

http://www.ed.mtu.edu/esmis/id65.htm

Xanthro

 

[partridge]

1.Light is white when it hits the water. At shallow depths red is filtered out then the rest of the rainbow. If all colors were filtered out then it would be black, the absence of all light. I agree with your instructor, otherwise how can you see anything at 100 feet.

2. Really depends on the size of a person's lungs doesn't it? 12 lbs seems a lot for an average though.

3. Depends on the water and the type of sound. Some sounds travel really far like the 10 miles but it probably doesn't sound like an engine anymore. The range of pitch will be cut by a lot. Think high pitched whirring versus the heavy chugging of a ship.

 

[RIOceanographer]

Technically, the water doesn’t absorb the light, particulate matter in the water absorbs the lights energy. Perfectly clear water would allow light to pass through.

I am being nitpicky because this is getting into my field, so please don't be offended if I correct you.

Your explanation is not completely correct. Pure water does absorb light. What you described with the long wavelengths being absorbed most strongly and the shorter wavelenghts less strongly is true, but those are absorption properties of pure water, not of the particles in the water.

That being said, particles and colored dissolved materials are also important factors in determing the optical properties of a water mass since there is never such a thing as "pure" water in the ocean. These things will have different absorption properties from the water and will change its color. For example if there are a lot of plankton with chlorophyll growing in the water, this can make the water look green. That is why the water off New England doesn't look like the water off of Hawaii. Depending on where you are, what is in the water will affect the optical properties.

Another optical propety is scattering. Scattering is when the light is redirected from its original path when it hits something. But that is a whole different story....

 

[eod]

"2. Another instructor said that an average human, when they fully expand their lungs, adds about 12 lbs of lift to their body. Does anyone know the volume change in the human body with a deep breath to see if this is true? It makes a great story for beginner divers, to teach them to exhale when trying to begin their descents, but I'd like to actually know if it's true."

Recently I did a test of my lungs capacity (vital capacity I beleive it is called). The result was 5l. The result was dead on the mark for average for people with my length and age. The other guy that did the same had almost 6l. From what I learned that day 5kg of lift can be added from completely empty lungs to completely full.

 

[Xanthro]

I am being nitpicky because this is getting into my field, so please don't be offended if I correct you.

Your explanation is not completely correct. Pure water does absorb light.

We could spill much ink discussing the physics of light. However, the discussion would be beyond the scope of the original posters question.

I fully understand that ions also absorb light energy, and that with enough depth, even pure deionized water would absorb energy.

That said, for practical purposes, it's not the water that aborbs light, it's particulants and ions. Since I'm unsure as to whether the original poster understands ions, I didn't mention it.

While there is no such thing as pure water anywhere, much less in the ocean, I'm sure you understand that "pure" deionized water behaves vastly differently than normal water. That was the point I was trying to make, without getting into ionization, or too technical. That's why I posted a link, so the orginal poster could learn as much or as little as desired to satisfy curiosity.

It's late and I'm tired, so my apologies for any spelling errors.

Xanthro

 

[RichLockyer]

1.Light is white when it hits the water. At shallow depths red is filtered out then the rest of the rainbow. If all colors were filtered out then it would be black, the absence of all light. I agree with your instructor, otherwise how can you see anything at 100 feet.

The instructor is correct ONLY in that the texts are incorrect as to the depths, but the ORDER of the loss of color is correct.

Conditions will determine how deep each color will go. I've been in water at 15ft and the red overlay on my drysuit is a cool violet.
Likewise, I've been as deep as 140 and the water in California is extremely green. Coryanactis Anenomes appear to be very, very pink at 100ft, but a light reveals that they are in fact orange. Red Gorgonians appear black, but a light reveals that they are a very bright red.

The breaks between the loss of colors is certainly not "every 5 feet", but then blue and indigo magically go deeper than people dive. Green is very much present at 100ft. Yellow does seem to look pretty "white" well before 70ft... the loss of green would depend on the shade. The light green on Oceanic scooters goes pretty light as well.

 

[RichLockyer]

2. Another instructor said that an average human, when they fully expand their lungs, adds about 12 lbs of lift to their body.

Exhale into a balloon.
One pint equals one pound of lift.

 

[RIOceanographer]

That said, for practical purposes, it's not the water that aborbs light, it's particulants and ions.

For the record, I am not trying to be argumentative. I am trying to be helpful. I really do this stuff for a living.

The absorption by the water itself actually is very important in the red wavelengths. To say it isn't is just not true. Water absorbs red very strongly and this is easily noticable to any diver.

Just to put a real number on it, a layer just 1 m thick of pure water would absorb 35% of incident irradiance at 680nm (red light). I got that right out of a textbook I had laying on my desk. This is why you can't see red at depth when diving unless you use your dive light.

What I think is confusing you is that in the blue/shorter wavelength part of the spectrum water is a very weak absorber and in that region you are right a diver would not really be able to notice its effect. Then the particles and dissolved materials are what you will notice like you suggest. If there aren't many of these things in the water it will look blue (like clear tropical waters). If there are lots of these things in the water it will usually look greenish or brownish (like in New England where I live).