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Color spectrum underwater.

  1. Feb 19, 2008 #1
    hi guys im new here.
    for my final year physics assignment i am testing how colors are absorbed under water at different depths.
    i am planning to go scuba diving and take photos of a color spectrum at certain depths
    as was wondering if anyone here new much about this and would be able to help me out with the physics behind this, and with some information

    thanks in advance.
  2. jcsd
  3. Feb 20, 2008 #2

    Andy Resnick

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    Water absorbs various wavelengths; stuff in the water scatters wavelengths as well. The absorption spectrum of water is available online:


    I'm confused what you mean by "take photos of a color spectrum".. can you elaborate?
  4. Feb 20, 2008 #3


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    Red is absorbed most easily, so, as you go deeper, you'll find less and less red light.

    - Warren
  5. Feb 20, 2008 #4


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    I think they mean take photos of a colour test card.
  6. Feb 21, 2008 #5
    yeah i do mean a color chart.
    im aware that red is absorbed first then orange and so on until violet goes.
    but what are all the physics principles behind this?
    as well what would be a good way of measuring the color change?
    any info you guys can give me would be appreciated.
    Last edited: Feb 21, 2008
  7. Feb 23, 2008 #6
    can anyone help me out with this one?
  8. Feb 23, 2008 #7


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    As far as I know, the colors with the longer wavelength are absorb first because the vibration resonant frequency of the water molecules is closer to the Infrared.
    try this link: http://www.exo.net/~pauld/physics/why_is_sky_blue.html
    If you wish to measure color you need a photospectrometer, but i guess you don't have one. You can take pictures of color cards at different depths (without flashlight of course) , and then use software like Photshop to check where this color is located on the color scale. for example, most of the red is absorbed on the first 6 meters so just take a picture of a red card every 1 meter and compare the pictures.
  9. Feb 23, 2008 #8
    yeah that is how i was planning on measuring the colors.
    but im not to sure on the theory behind it?
  10. Feb 23, 2008 #9


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    One of things you'll have to be careful of is getting accurate images from your camera. Both film and digital cameras go to great lengths to try to normalize the images for you. It will mess up your tests unless you take pains.
  11. Feb 24, 2008 #10
    thanks for all your help so far guys
    so what are pains then?
    and also if there is any more theory behind this you could tell me about?

    edit: another question, shorter wavelength colors like blue are scatter more easily. does this help out in why blue can be seem at greater depth, because it is scattered and therefore scattered in all directions include deeper. or is this not correct.
    Last edited: Feb 24, 2008
  12. Feb 24, 2008 #11
    About theory, my idea is; light has an energy and in material environments loses it. Since red has lowest energy in visible spectrum, will lose energy first.
  13. Feb 24, 2008 #12


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    I'm confused. You're in the final year of Physics, and you're asking fairly basic questions about light. Can you tell us a little more about your experience so we can understand how to help you?
  14. Feb 24, 2008 #13
    im in final year of high school physics, and we haven't done much on light in class yet.

    so yeah what is pains? and if you could help me out with the scattering it would be nice?

    as scattering causes the high wavelength colors like blue to be scattered more easily. how does this help blue be seen at greater depths?
  15. Feb 25, 2008 #14


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    It doesn't.

    I would bet that scattering is a very small factor compared to preferential absorption.
  16. Feb 25, 2008 #15

    I suspect that the absorbtion of energy may be more dominant that scattering. Absorbtion would be more like the molecules of water consuming some energy into vibrating molecular bonds. Scattering would be more like a re-direction of light so that it does not get to you down below.

    As for your experiment, I think that one of the key things to make sure you do before you go down, is to understand what you are going to use for a response. Since the camera may try and adjust lighting conditions, you could cannot rely on it as an "absolute" reference, meaning if you were to photograph a single red card by itself at various depths, you don't know how the camera is going to adjust exposure time on it's own. Instead, you could use a metric which compares the intensity of a red sample to a blue sample within the SAME photograph, at various depths, and take a percentage of one from the other. You could use the RGB values in photoshop for this. For example, here is some made up data:

    Depth Red Value Green Value Blue Value Red as % of blue Red as % of green
    0 225 235 235 =225/235 =235/235
    90 20 50 200 =20/200 =50/200

    This gets you half way there. To take the experiment further (which may not be necessary), you would want to also construct the same table in similar daylight lighting, out of the water, but use varying camera exposures (instead of depth) to make sure that the relationship between red, green, and blue don't change dramatically with just lighting intensity. With that, you can get a complete picture of how the colors on you color card are changing RELATIVE to each other. Again, an absolute measurement with your setup may be difficult. Go for relative. A final note is that if you can control your camera shutter speed and aperture, it may help to keep this all constant so you don't have to consider that variable.

    Hope this helps (and wasn't obvious)
    Last edited: Feb 25, 2008
  17. Feb 26, 2008 #16
    thanks bchung that really what i was after
    just not to sure what you mean about using a metric
    i was planning on taking a color chart with like 10-20 colors on it.
    as well as one with the full spectrum of colors and a Grey scale chart.
  18. Feb 26, 2008 #17
    If i want to do an experiment about this, i suppose prefer to do this in lab. In a dark room, a bulb, a prism, diffent wideness of aquarium and a camera.

    I don't guess the result but red light must lose intensity more. I suppose this may compare with polaroid photos. Else, i would try to do this with another liquid. I suppose easy to try. It would be better to have more sensitive devices then a camera but... only an idea. Wish you luck.
  19. Feb 26, 2008 #18


    By "metric," I simply mean a measurement you can put a number on. So, when you finally have your color charts, what do you plan to do with them? How do you get a number out? For your report, you could simply include all the photos and make a case that the red looks "more" or "less" intense at a certain depth, for example, and that may be fine. What's even nicer is if you can quantify how much more in some way.

    For example, you could import your pictures into photoshop. Then, you could take a red reading from the each red square at each depth, and a blue reading from each blue square at each depth. An example of a metric would be the ratio of red to blue, which is a single number that you could plot on a chart, giving you a curve of red/blue ratio vs. depth. If one color is getting absorbed faster than another, you would expect some kind of change in the ratio.

    So, do some thinking on your metric. Think about the objective of your experiment, and find a simple metric that would nail your objective squarely.

    Very interesting experiment. Sounds like a fun way to mix physics and a great outdoor activity.

  20. Mar 17, 2008 #19
    just a few more questions about this guys
    do you think that pressure or temperature will vary the results
    as the deeper underwater the more pressure, will this have any effect
    thanks for any info you can give me
  21. Mar 17, 2008 #20
    i suppose pressure may not effect directly but temperature will vary in water. Thus, different density of water may effect. Most of the time upper is more hot.
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