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Reflection with a complex material

  1. Nov 17, 2008 #1

    I am currently looking into refractometry, and how light reflects off a interface between to materials. I have looked through most of the other threads but nothing really explained what I am looking for.

    My problem, I have a digital refractometer and I am measuring the reflective angle of plant sap through its internal glass prism. However, plant sap contains a water-surcose solution, with free compounds like protiens, minerals, cellulose etc. Because of this I am not sure if my refractometer index readings are a function of the water-soluble content (water, surcose, fructans) or the water-soluable and structural content (+ the proteins minerals etc.).

    If the incident light is in a perfect material, say glass, and hits a second material which has many different types of compounds and atoms in it, what happens to the reflected light and its angle?
    I notice that you should treat the material as a whole structure when photons hit it. But if this material has many different atoms, molecules and structures at this interface you can't apply this simple rule.

    Thanks in advance and any ideas would be greatly appreciated.
  2. jcsd
  3. Nov 17, 2008 #2
    One thing I should add,

    The refractometer is finding the refractive index of the sap by reflecting light off it onto a small CCD sensor. From the critical angle of the sap glass interface, a shadow is cast over the CCD and it finds the refractive index from measuring this amount of pixels lit up by the light.
  4. Nov 18, 2008 #3

    Andy Resnick

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    As long as the transverse size of the illumination is large enough (more than a few hundred microns), you can consider the solution to be homogeneous.
  5. Nov 18, 2008 #4

    Your digital refractometer works on the critical angle basis. Light incident on the boundary layer of the prism and air will be totally internally reflected. However when the air at the boundary interface is replaced with a fluid (with a refractive index lower than the refractive index of the prism), some of the light will be reflected off the boundary interface towards the linear array of photodiodes and some of the light will be lost. As you stated, the portion of light that is reflected forms a shadowline on a linear array of photodiodes. The relative position of the shadowline is representative of the refractive index of the fluid.

    You can use Snells’s Law to calculate the interaction at the boundary interface. Most handheld digital refractometers use a standard glass prism made of BK-7 glass with a refractive index of ~1.51673 @20°C. MISCO Refractometer uses only sapphire prisms which have a much higher refractive index.

    Concerning your question on fluid composition, on the most basic level, a refractometer really just measures total dissolved solids. As Andy Resnick said, you should consider the solution to be homogeneous, meaning there is one uniform refractive index throughout the solution. For your purposes consider the dissolved solids as being additive to the total refractive index of the solution.

    In the case of plant sap, it is likely that the majority of the dissolved solids will be sucrose with a much smaller contribution (almost negligible) from the other components. The other components would have to be in an equivalent concentration of greater than 0.1% sucrose by weight before they would even change the refractive index by 0.0001 nD20.

    Good Luck!

    MISCO Refractometer
  6. Nov 18, 2008 #5
    Hey Andy and Mrainer, thanks for the help i appreciate it !

    Andy, I would assume the enzymes and protiens would be much larger than the wavelength, wouldn't this mean the photons that hit these compounds would have a different critcal angle, and hence I would obtain different readings ?

    I realise this extra compound content should have a minimal effect, however, simply rolling a grass sample before it is crushed increases the refractometer reading (2% brix (not rolled) upto 10% brix (rolled for >1min)) I assume by doing this I am breaking up the cell wall of the sample and this is introduced to the sap, which in turn increases the reading. Even rolling the sample for 5 seconds will increase a reading by ~1% brix. So i'm guessing these cell wall compounds are large, and affecting the brix reading.

    I guess what I am asking is even when I do not roll the sample, the sap will contain solids which noticably affect the refractometer reading?

    Could you give me a more detailed description of exactly what is happening on an atomic scale, with an interface which cotains surcose water and many larger compounds and solids, in terms of photons and their interaction with this solution. I just want to understand what is physically happening in a situation like this.

    I would have also thought that larger compounds would begin to settle on the bottom of the well on the refractometer, hence more solids would be on the interface, changing the reading...

    Thanks again for your help
  7. Nov 19, 2008 #6

    Andy Resnick

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    Proteins are at most several nm in size. Typical wavelengths used in (visible) optics is around 500 nm. Proteins are essentially point particles for visible light.

    You are dealing with a instrument that measures a macroscopic material property- be careful trying to extrapolate to a microscopic description.
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