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Why orange color is visible with blue LED?

  1. Nov 2, 2015 #1
    I am making a DIY spectophotometer.

    What you see in this photo is flask of methyl orange solution in water with blue led in the back in a black box. As you can see the surrounding is blue because the box reflect blue light, however the light coming through the flask is orange. I don't understand why, blue LED gives narrow spectrum of blue light (450-500 nm from wikipedia). So there should be no orange color (600 nm or mix of green 500-570 nm and red 610-760 nm). Thank you.
     

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    Last edited: Nov 2, 2015
  2. jcsd
  3. Nov 2, 2015 #2
    Do you know whether the orange is a reflection or fluorescence?
     
  4. Nov 2, 2015 #3
    Ohhh. I never knew that methyl orange have fluorescence. From the photo, I think it is reflection, because there is a dark area in the solution, if it is fluorescence that dark part must be lit up a bit.
     
  5. Nov 2, 2015 #4

    NascentOxygen

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    The flask is filled to the 50ml level with water plus a few drops of methyl orange?
     
  6. Nov 2, 2015 #5
    100 ml of deionized water and 0.01 gram of methyl orange powder and then half it.
     
  7. Nov 2, 2015 #6
    I understand that LED is monochromatic light, but the photo shows that there are orange light. Or I misunderstood something?
     
  8. Nov 2, 2015 #7
    I think the methyl orange, has a large portion of it's absorption spectrum in the blue range,
    That energy, once absorbed, has to go somewhere.
    It is just a guess, but one of the spontaneous decay products, could be your orange photons.
     
  9. Nov 2, 2015 #8

    Andy Resnick

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    Methyl orange is fluorescent- it absorbs blue light from the LAD and emits over a fairly broad waveband, including orange (540 nm):

    https://www.researchgate.net/publication/269036448_Absorption_and_Fluorescence_Spectra_of_Methyl_Orange_in_Aqueous_Solutions [Broken]
     
    Last edited by a moderator: May 7, 2017
  10. Nov 2, 2015 #9

    NascentOxygen

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    The orange light seems to emenate from the first few mm of solution at the base---that first encountered by the laser light. So the blue is rapidly absorbed and doesn't penetrate far at all? The orange pattern in the centre of the image is a complex reflection of that orange glow at the base? At the concentration of dye involved here, in white light would the solution have a pale appearance resembling light beer?
     
  11. Nov 2, 2015 #10
    The 5 orange circular thingy is LED and the rest is cause by refraction and reflection.

    So if I use diffraction grating film, I will see two spectrum (orange and blue) right?
     
    Last edited: Nov 2, 2015
  12. Nov 4, 2015 #11

    NascentOxygen

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    It would be interesting to see the blue LED behind a wedge of the dyed liquid, e.g., by tilting a half-filled flask. Where the wedge of liquid is thick, we'd see this orange light, where the wedge is zero thickness the transmitted light will be blue, and maybe there'd be a level where the light coming through would be a mixture, viz., green? Kevin_tee, do you have the apparatus that you can try this?
     
  13. Nov 4, 2015 #12

    Andy Resnick

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    I suspect that if you observe the totally-internally reflected 'images' of the LED, you will see two spectra- one from the unabsorbed blue light and one from the fluorescently emitted orange light.
     
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