You just go through the paper link attached here........... your confusions are discussed in the paper below

doi:10.1088/0953-8984/18/13/002

You can combine Fresnel's equation and Snell's law, see any elementary book on optics or e.g. http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/freseq.html.

kindly go through papers references.........and elementary book as suggested by Tapio Ala-Nissila sir

Kindly go through with the following reference

http://www.opticsinfobase.org/josaa/abstract.cfm?id=2164

Your transmission spectrum and film thickness have to be used in the analysis. Check if your transmission pattern has a series of maxima and minima. Then by carefully finding the maximum and minimum intensities and use the fourlayer analysis air, substrate, film, air you can get n, band gap(if your sample is semiconductor), and thickness. We use this procedure in our Solid State Laboratory for MSc students.

Pl use the following reference

Determining refractive index and thickness of thin films from wavelength measurements only

R. Swanepoel

JOSA A, Vol. 2, Issue 8, pp. 1339- 1343 (1985)

http://dx.doi.org/10.1364/JOSAA.2.001339

Abstract

The dispersive refractive index n(λ) and thickness d of thin films are usually determined from measurements of both transmission and wavelength values. Many factors can influence transmission values, leading to large errors in the calculated values of n(λ) and d. A method is presented to determine n(λ) and d in the region where k2 « n2 from wavelength values only. This entails obtaining two spectra, one at normal incidence and another at oblique incidence. The method yields values of n(λ) and d to an accuracy better than 1%.

Good luck

Thanking u all for your valuable suggestions