Question

What is the formula to find the absorption coefficients of materials?

Well, according to 'principles of optics' from Born & Wolf (p614) the absorption coefficient formula is the following:

X=(2Wnk)/c=(4Pik)/L

Where x is the absorption coefficient,W the frequency of light in the vacuum, c the speed of light in vacuum, l is the wavelength inside the medium and n and k are the refractive index and the attenuation index (also called extinction coefficient) respectively. The quantities n and K may be easily be expressed in terms of the material constants (dielectric constant, the electrical conductivity and the magnetic permeability)

Note that this expression of the absorption coefficient is a good approximation if the dielectric constant is real and the excitation frequency is low (long wavelength)
Answered by: Alex Bouhelier, Ph.D Physics, Uni Basel (CH)

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