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Something that absorbs at longer wavelengths looks more red?

  1. Apr 29, 2012 #1
    So the colors we perceive are a result of photons of a certain wavelength being reflected back at us. So why in a recent experiment I did, did nanoparticles which had an absorption band that red shifted as the reaction proceeded, look more and more red from an initial yellow color?

    If the particles were absorbing light more and more at longer wavelengths shouldn't they look less red since those photons are being absorbed? Although I also did an emission spectrum which showed the emission band maxima increasing as well. Other than small effects like Stokes shift, what is the difference between all light being reflected and some light being absorbed and then emitted? If there wasn't a difference wouldn't everything just look white?

    tl;dr- why did my particles look red as time went on if the wavelength of their absorption peak increased?
  2. jcsd
  3. Apr 29, 2012 #2


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    What exactly caused the absorption band to change?
  4. Apr 30, 2012 #3


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    How do you measure absorption? By the light which passes through your object, or the light which is scattered (more general: radiated in some other direction)?

    Maybe your particles absorb the red light and emit it somewhere in another direction afterwards. In this way, they look red, but only a smaller fraction of the red light passes your sample.

    Do you have full spectra of the red sample and the not-so-red sample? Maybe it is just some effect from the sensitivity of human eyes.
  5. May 1, 2012 #4
    The growth of the diameter of the nanoparticles. This changes the quantized energy of the electronic transitions for a spherically symmetric system based on quantum confinement.

    E= (n2 h2)/(8me R2 )

    As you can see, bigger radius means lower energy means longer wavelength absorbance. So why does this correspond to looking more red?
  6. May 1, 2012 #5


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    I'm not familiar with this, but if true, then I don't believe it should look red. Are you certain that the material is absorbing longer wavelengths of light and not reflecting them?
  7. May 1, 2012 #6
    Do you understand what make their initial yellow color?
    It may be that what you see is some sort of fluorescence?
    What kind of excitation is used? White light or some narrow band light? Or it is maybe UV?
    If they absorb light of a specific color, they will emit it too, when they go back on the ground state. Unless there is a two or three step process.
  8. May 1, 2012 #7

    Ken G

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    As others have said, it sounds like you are looking at reflected light. Then you are seeing something a bit like why the sky is blue-- the particles in the sky interact better with blue light, so when you look directly at the Sun near sunset, it looks red. That's like the absorption you expect from your nanoparticles (only that's not what you are seeing). When you don't look directly at the Sun, you are seeing scattered light, and that's the light that does interact with the sky "absorption", only it isn't pure absorption, it re-radiates and looks blue.
  9. May 1, 2012 #8
    Yes I am. In the experiment I took a UV-vis spectrum of the reaction at different points in time. The band maxima in the spectrum increased in wavelength as time proceeded (as the nanoparticles grew).

    Well, in addition to an absorption spectrum I did an emission spectrum over time and saw that wavelength increase as well.

    So what is the difference between reflected light and absorbed and then re-emitted light? It should look the same right (other than minor effects like Stokes shift)?

    Yeah, it must be something like this.

    So, to bring the discussion to a more general topic, you know how people always say that things looks a certain color because some light gets absorbed and we see the reflected light that isn't absorbed? This isn't right is it? If light is absorbed it is subsequently re-emitted. Hence my above question about the difference between reflected and fluoresced light. So the colors we see are really the result of scattering?

    (or I guess it could also be that the absorbed light is given off as heat and we really don't see those wavelengths)
  10. May 1, 2012 #9
    What is UV-vis spectrum? Are you illuminating the sample with ultraviolet light?
  11. May 2, 2012 #10


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    If the light is actually being absorbed, then being re-emitted after a short amount of time, that is not reflection. Sounds more like luminescence.

    Absorbed light from the visible range typically heats the object up. UV radiation, depending on the wavelength and the properties of the object, can result in breakdown of the material, luminescence, increase in heat, etc. Luminescence is the absorption of UV light which increases the energy of some of its electrons temporarily. After a certain amount of time the electrons fall from their temporary states and in the process release the energy in the form of visible light.
  12. May 2, 2012 #11

    Ken G

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    Right, often the term "absorbed" is reserved for if the energy is ultimately thermalized (turned into heat and re-radiated in the IR), and "scattered" is used if the light is absorbed only very quickly but then immediately re-emitted at more or less the same frequency at which it was absorbed ("Rayleigh scattering" is if it is re-emitted at exactly the same frequency). So if some of the light gets "absorbed" and some gets "scattered", we see the scattered part and associate that with the color (but in some sense it always has to be absorbed, at least instantaneously, or else it would just pass right through-- like how a traffic light acquires its color, or the setting Sun). So it's not just the substance that matters to color, but also the geometry of the illumination (e.g., is "air" red or blue?).
  13. May 2, 2012 #12


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    What are your particles made of? What's their mean size?
  14. May 3, 2012 #13
    CdSe nanoparticles, I think they are pretty small, maybe 2-5 nm radius.
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