Fish tank optics

Discussion in 'Physics' started by ErnieM, Jun 17, 2016.

  1. ErnieM

    Thread Starter AAC Fanatic!

    Apr 24, 2011
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    Something strange is going on in my all glass aquarium.

    It is a rather large tank, four feet by maybe twenty inches tall. I had a nice printed backdrop on it that never looked right as sects seemed to be missing some sort of overcoat. Some parts were bright, some were dull.

    When I moved the tank I got a new backdrop. As I was tossing the old one out I noticed it looked just fine in my hands. Oh well, put the new one in and son of a gun the same thing is happening!

    This time in the new position I can look thru the side of the tank. Most of the real glass is bright, just reflecting ambient light. But some areas you can see the backdrop pattern, and those same areas are the portions that appear very brightly. It also seems like these bright portions are trapped against the glass, perhaps by a thin coat of water, or just electrostatic attraction.

    Anyone know what I am seeing? Is there a way to get the entire backdr to be as bright as the rest?
     
  2. Alec_t

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    Sep 17, 2013
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    Is the printed backdrop in contact with the outside face of the glass, or spaced from it by an air gap?
     
  3. nsaspook

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  4. KeepItSimpleStupid

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    Mar 4, 2014
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    Lighting, shadows, possibly?

    Consider a translucent background lit through the glass.

    I had a small tank (10g) at one time and the background was sort of 3D plastic
     
  5. ErnieM

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    Apr 24, 2011
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    The bright portions are tightly against the rear glass as if they were glued by a thin water layer or electrostatics.

    Give it the slightest gap and the brightness goes away.

    Also, when viewed from the side only the areas in contact are seen. Otherwise it is a whiteish appearance with no detail from the backing.
     
  6. GopherT

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    Nov 23, 2012
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    The answer is "refractive index differences and Fresnel Equation".

    The gap between glass and image must be traversed twice and you get attenuation both times (when there is a gap). There is effectively no gap and no refractive index mis-match when the image is pressed into the glass.

    The greater the mismatch in refractive index, the greater the loss by reflection vs transmission through the glass. Notice the loss in intensity upon transmission.

    [​IMG]


    https://en.m.wikipedia.org/wiki/Fresnel_equations

    https://en.m.wikipedia.org/wiki/Refractive_index

    Also, look here, the attenuation of a single pass is noticable, now think about how that attenuated light will be further attenuated by only reflecting the colors of the image and then again attenuated by the fresnel effect as that light comes back into the aquarium. Not much is left.

    [​IMG]
     
  7. Alec_t

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    Sep 17, 2013
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    Agreed; refraction and total internal reflection account for the patchy effect. If you could find a gel which didn't damage the backdrop you could use that to 'stick' the backdrop to the glass and eliminate the air layer which is causing the problem.
     
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