Dear  Dr. S. A. Hejazi

Al Salamu Alekum

Generally, the absorption coefficient (alpha) is related to photon energy (h*neo) by the known equation as:

       α = β/(hυ) (hυ - Eg)1/n        Or          (αhυ)n =β(hυ - Eg)                 

where β is a constant called the band tailing parameter, Eg is the energy of the optical band gap and 2 is the power factor of the transition mode, which is dependent upon the nature of the material, whether it is crystalline or amorphous. According to Tauc's relation:

The plotting of  (αhυ)n Versus the photon energy (hυ) gives a straight line in a certain region. The extrapolation of this straight line will intercept the (hυ)-axis to give the value of the optical energy gap (Eg).

Therefore, the intersection value (in eV) is your band gap energy according to Tauc's model.

With respect to the appended snapshot, Lambert's law is given as:

I=IoExp (-Alpha*Thickness), So 

Alpha*Thickness = ln (Io/I)

Hence, Alpha = (1/d) ln (Io/I) = (1/d) ln A = (2.303*A)/d

I hope this help you.

Good luck 

For more details Please go to:

https://www.researchgate.net/publication/281495242_Influence_of_composition_on_optical_and_dispersion_parameters_of_thermally_evaporated_non-crystalline_Cd50_S50-xSex_thin_films?focusedCommentId=58bfc21f82999cd4be0cb478

https://www.researchgate.net/publication/283338024_Effect_of_Se_addition_on_optical_and_electrical_properties_of_chalcogenide_CdSSe_thin_films?ev=prf_high

https://www.researchgate.net/publication/304150361_Study_of_optical_properties_of_thermally_evaporated_ZnSe_thin_films_annealed_at_different_pulsed_laser_powers

I hope they help you.

Good luck 

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