See R.G. Burns (1993) Mineralogical applications of crystal field theory, Table 3.2, p.47:

Absorption coefficient or Extinction coefficient alpha=log(I/Io)/d, cm-1

Molar extinction coefficient epsilon = log(I/Io)/Cd, litre mole-1 cm-1

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Io and I are the intensities of incident and emergent light, respectively; d is the thickness, in cm, of the material or optical path length in the medium; C is the concentration of the absorbing species in moles per litre or gm ions per litre.

Michail N. Taran sir, but I have a powder sample. Is the above equation is applicable in my case?

Optical absorption coefficient cannot be determined from a powder diffusion spectra. The theory shows that in that case one can obtain only alpha/S, where S is coefficient of dispersion: see e.g. Judd and Wyshezci (1975).