A detailed study of the dependence of optical constants on thickness for vacuum-deposited hydrogenated amorphous silicon thin films is reported. The thicknesses of the films range from 190 nm to 540 nm. The refractive index n(λ), absorption coefficient α(λ), extinction coefficient k(λ) and consequently the band gap, are determined from the spectrophotometric measurements of the film transmittance in the wavelength range 400-2500 nm. The average gap Ew is calculated as well using Wemple's equations. A comparison between different mechanisms to study the band gap and their correla1 tion to the obtained results is given. The carrier concentration N/m* , static refractive index no and the high frequency dielectric constant ε∞ were studied.