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Optics problem --- absorbtion index

  1. Jun 10, 2015 #1
    I have an optic problem

    I want to calculate absorbtion index

    I know transmittance by 90% ;refraction index to material 1.586 ; thickness 2 mm ; power of source 21 lm
  2. jcsd
  3. Jun 10, 2015 #2


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    The transmittance of a weakly absorbing slab can be approximated by

    [tex]T=\frac{(1-R^2) e^{-\alpha d}}{1-R^2 e^{-2\alpha d}}[/tex]
    and α is the absorption coefficient. It is related to k, the imaginary part of the complex refractive index : ##\alpha = \frac{4 \pi k }{\lambda }##
  4. Jun 11, 2015 #3

    thank you for your answer but you are not specific
  5. Jun 11, 2015 #4


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    What is not clear? no is the refractive index of the ambient (air). n is the refractive index of the slab, d is its thickness. λ is the wavelength. k is the extinction coefficient. If you know the transmittance, the refractive index and the thickness, and taking no=1, you can calculate α. If you know α and the wavelength, you can get k. I think, it is you want to calculate.
    You need to measure the transmittance with as high accuracy as possible. At least 0.1% accuracy is needed.
  6. Jun 11, 2015 #5
    now is clear thanks
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