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Guide To Selecting Refractive Index

Source: HORIBA Instruments, Inc.

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Technical Note: Guide To Selecting Refractive Index

Refractive index is a physical property of a material that can have a significant effect on light scattering particle size measurements. As such it is important to have an understanding of what refractive index is and how to make the correct selection for the material to be analyzed.

Refractive Index is defined by two components - real and imaginary RI = n – ik, where:

  • n = the real component, which is the ratio of the velocity of light in a vacuum to the velocity of light in the material
  • k = the extinction coefficient of the material
  • i = √ - 1
The real component can be determined from published tables or it can be measured using Snell's Law: n sin θ = n' sin θ', where:
  • n = refractive index (RI) of first substance (usually air)
  • θ = angle of incidence
  • n' = refractive index of second substance (usually measured substance)
  • θ' = angle of refraction (deviation from original direction
The imaginary component (k) of the refractive index is the extinction coefficient of the material, defined as the reduction of transmission of optical radiation caused by absorption and scattering of light k = ( λ / 4π ) α , where:
  • k = the extinction coefficient
  • α = the absorption coefficient
  • λ = the wavelength of light used

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Technical Note: Guide To Selecting Refractive Index