Dear Sanusi Abdullahi,

Look over the following data:

https://www.researchgate.net/post/How-to-calculate-the-thin-film-thickness-using-the-envelope-method-by-absorbance-data

https://www.researchgate.net/post/Is_there_any_free_software_that_can_calculate_optical_constants_from_thin_films_reflect_and_or_transmetance_spectra_data

https://www.researchgate.net/post/Using-envelope-method-to-determine-the-optical-constants-of-thin-film

https://arxiv.org/ftp/arxiv/papers/2011/2011.04956.pdf

Dear@ Sanusi Abdullahi

Have a nice time and good day!

Please read these papers, you will find what you want:

Article Investigation on dispersion parameters of Molybdenum Oxide t...

Article Effect of thickness and post deposition annealing temperatur...