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I Change in energy as light passes through air/water

  1. Jul 27, 2016 #1
    Okay, best to provide some background here. I am trying to understand the light coming from the LED's over my aquarium. I have blue moon lights I think at about 540nm and 450nm. Oddly enough, when they are on, there is a green tinge to the light. I had assumed that this was because the light lost energy as it hit the water, therefore lengthening the wavelength. However, as I tried to further prove to myself that this was what was happening I learnt that the wavelength of the light decreases as it hits the water.

    Up until now, I was of the understanding that the UV light was higher energy, but there is a contradiction in the above. Is it that the UV light is a more 'concentrated' energy, and that the colour change is due to refraction and / or 'wave-spreading' of the more angular incidences of light (they are only 60 degree lights)? Or is it that the light loses colour temperature?

    I am guessing that it is a combination:
    - the green tinge is from wavelength being spread as it hits at an angle, combined with some prismatic effect bending green light, present in the imperfect light beam, further than it bends the blue.
    - The light also loses energy at the surface and as it dissipates through the water.

    Please confirm / correct / elaborate
  2. jcsd
  3. Jul 27, 2016 #2


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    I don't know enough about this stuff to answer your question but I can tell you for sure that the range you have is the range of UV that is the very best for causing many substances to fluoresce green or greenish yellow. Possible that's adding to the effect you are seeing (something in the water is fluorescing ?)
  4. Jul 27, 2016 #3


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    Light comes in quanta called photons. Each photon has an energy equal to ##h\nu## that is Planck's constant times frequency. The total light intensity is number of photons times energy of each. As light enters the water, its frequency does not change, it still has the same energy. A certain portion of photons will be reflected at the surface, and as they travel through water, they get absorbed. Neither of this processed changes the energy of individual photons, just their number.
    There is also light scattering, that changes of direction of light. (This is what makes the sky blue and sunset red).

    540 nm is actually green light. Your LED produces both, blue and green.
    The greenish tinge could come as a results of one or more of the following:

    Stronger absorption of blue light
    Stronger scattering of blue light

    Fluorescence occurs when something absorbes a photon of one energy and re-emits a photon of a lower energy.
    Fluorescens is quite common when incident light has short wavelength.

    And yes, green light will refract at a slightly different angle then the blue light when entering water.
  5. Jul 27, 2016 #4


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    The colour we see is due to the frequency of the light, which does not change. It is the wavelength that changes through different media because the speed of the light is less than c, the speed in empty space. We can safely say that, in these circumstances, there can be no change in the frequency of the light. The wavelength changes as light goes through different media revert to the 'in air' wavelength when it emerges again and when you see it.
    If you are getting an apparent change in colour it can either be due to filtering of some of the wavelengths in your lights or fluorescence due to some UV component of your lights and something in your water or the tank walls.
    Thick cheap glass has a greenish tinge when you look through the edge of a sheet but your aquarium glass will be nicer quality than that, I would think.
  6. Jul 27, 2016 #5


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    I have a Marineland model 01G33 fish tank light fixture, with a blue LED moonlight setting.
    Visually, and photographed against the white lights, my blue LEDs, run though a 1000 lines/mm diffraction grating, show that they are very, very dirty little lights.
    By that, I mean that they emit all the colors of the rainbow, from violet to red.
    Of course, they are brightest in the blue region.

    Upper spectrum is from a blue bulb.
    Lower spectrum is from a white bulb.​

    ps. If there's a green tinge to your water, then it's time to change the water.
  7. Jul 27, 2016 #6
    Sorry, its 450nm and 465nm on the moon channel.
    Photo below (strong algae is a sign of good health, although poor nutrient balance which is what I am supposed to be working on :)

    Yes, I think the front glass is low iron (mass production ik).

    Looks like it must be fluorescing ... amazing ... the organics and ionics in the aquarium.
    I put it down to fluorescence as it is cloudy (rather than rayish, for want of a better word), the blue light nearer the bottom (blue light is known to penetrate deeper due to the higher energy) might eliminate 'scattering' as the major factor. Absorption, and fluorescing ... amazing.

    I think I recall that the light does include some UV in its spectrum.

    Thank you so much for your responses.

    For bonus points ... Would it be the short wavelength UV fluorescing (getting chopped) and changed to green, or would it be the the whole spectrum moving along, to a longer wavelength.

    Attached Files:

  8. Jul 27, 2016 #7
    OmCheeto, I suspected as much. quite a spread for those high efficiency LED's.
    Mine are Cree 3W LED's, not sure about the marineland ones but I suspect they are similar.
    Actually great image to see. I am not sure if you are aware, but there is a reason the aquarium lights have intensity spots like that, and its called Photosynthetically Active Radiation (PAR), It is the bands of the spectrum that plants use to photosynthesise. Never seen it like that. Brings new fuzzier to memory of my first year physics prac. I have attached the curves from my manufacturer.
    As you mention green water, please let me clarify that yes, I am in the process of changing from those dirty 'lignosulfonate' chelates to the purer DTPA / EDTA forms for my aquarium, and cancelling the use of that precipitous MNSO4. I am getting there, give me a break!


    Attached Files:

  9. Jul 28, 2016 #8


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    It will not be monochromatic. Those figures will relate to the peak wavelengths.
  10. Jul 28, 2016 #9


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    Actually, according to everything I've read over the last 4 hours, the LEDs are very monochromatic.

    Cree apparently makes only one model of blue LED: Royal Blue.
    My spectral analysis may be boogered by the deficiencies of optical Bayer filtering in dollar store digital cameras.
    Unfortunately, I don't have access to high dollar spectral equipment.

    Perhaps I should box up my fish tank lamp, and send it to @Andy Resnick for proper analysis. :biggrin:

    ps. There was talk of "remote phosphor LEDs" at the one [graph] website, so I thought maybe Cree put some phosphors into the mix.
    So I took my UV light, and placed it over the lamp, and only the white LEDs glowed.
    I could also see the my backup optical sensors, aka "my eyes", had fooled me into thinking all of the LEDs looked the same.
    I could clearly see that there was no visible phosphor covering the blue LEDs.
  11. Jul 28, 2016 #10

    Andy Resnick

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    I'm surprised your measured LED spectrum is so broad- if you send the light through the grating and project against a wall instead of the camera, does the spectrum look that broad to your eye?
  12. Jul 28, 2016 #11
    No, this is incorrect. Some energy is lost because some photons are absorbed, but the energy in each photon stays the same. There are just fewer photons making it all the way through.
    Also, the energy of a photon depends on the frequency of the photon, not the wavelength. In vacuum, the wavelength is just inversely proportional to the frequency, so you can say longer wavelength <=> lower frequency <=> lower energy. But if you are crossing between different materials, the wavelength will change, and the frequency and energy will stay the same.

    The technical term for 'wave-spreading' is "dispersion". The color change is due to both dispersion and absorption. Dispersion will result in the color changing as you look at different angles and areas, but absorption will filter the color.
  13. Jul 28, 2016 #12


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    One moment, please.....

    [an hour later]

    Oh. My. God.

    Bathroom science.......
  14. Jul 28, 2016 #13


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    How were you thinking of spectral analysis with Bayer filtering?
  15. Jul 28, 2016 #14


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    Dollar store scientist?
  16. Jul 28, 2016 #15


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    Tell me a bout "dollar store scientist".
    The analysis in a camera is very similar to the analysis by the eye (hardly surprising). It uses just three wide band analysis curves which cannot tell the difference (and nor can your eye) between a monochromatic source and a wideband source with the same colour (metemeric match - see link). If you actually know that the source is monochromatic then three RGV values can identify the wavelength but you need to know it's monochromatic before you can do that.
    But I could accept the results of Andy [EDIT with apologies: damned autocorrect!!!] Resnick's suggested spectroscope as evidence one way or the other.
    Not long ago I bought a cheapo spectroscope (sold mainly for lapidary identification). It is great fun to examine (qualitatively) the spectra of many light sources. It could be good for some optical filters too.
    Last edited: Jul 29, 2016
  17. Jul 28, 2016 #16
    Of course. Thanks Khashishi.

    have been reading From LED lights
    Revised Spec - Optical power: 40 pcs 3W Bridgelux Bxce/Bxcd LED
  18. Jul 28, 2016 #17


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    Took a nap, as doing such usually reboots my brain.

    I've decided there's no way for me to inexpensively focus the light from the bulb. ie, less than a whole roll of duct tape.
    I've also decided I'm chasing a ghost. I have no idea who made my blue LEDs, nor what their spectrum should be.
    It's tkyoung who has the Cree lamps.
  19. Jul 29, 2016 #18
    OmCheeto, My Aquarium Light is a DSunY, with Bridgelux LED's. Sorry for the confusion. Did you check the spectrum without the filtering? Did it change?

    I have just done a 50% water change, including makeup salts, and there is a significant change in fluorescence. This is an awesome learning. Not only does it show the amount of organic buildup, but with a bit more research I might be able to pinpoint the organic by the colour of the light. Thank you so much!

    Before photo:
    [NOPARSE][PLAIN]http://www.gentlespring.com.au/20160728_095436.jpg[/NOPARSE] [Broken]

    After photo:
    [NOPARSE][PLAIN]http://www.gentlespring.com.au/20160729_164242.jpg[/NOPARSE] [Broken]

    Example Luminescence: Absorption/Emmitance Spectra (mine is not this one, being green)
    [NOPARSE]http://www.gentlespring.com.au/Screen [Broken] Shot 2016-07-29 at 5.20.45 PM.png[/PLAIN] [Broken][/NOPARSE]

    Last edited by a moderator: May 8, 2017
  20. Jul 29, 2016 #19
    Interesting. A picture is worth a thousands words. Yes, it is possible for fluorescence to create photons of lower energy. Maybe that's happening here.
  21. Jul 29, 2016 #20


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    Here's a somewhat more laymanish site that describes the effect:

    Red fluorescence of algal cells is activated by excitation with blue light. This signal is weak when compared to the scattered green light and therefore not usually visible by eye. [ref]

    I'm an expert in laymanology.
    Oh! And something I might be able to do!

    Measuring water fluorescence
    Unlike apparent characteristics of water, such as its colour and transparency, fluorescence is not usually visible by eye when looking at the sea or a lake. This is due to the weakness of the signal, when compared to the green backscattered light. To make fluorescence visible we have to reach into our bag of tricks.

    I've an appointment with the river in 30 minutes. Perhaps I'll collect some indigenous algae samples.
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