Light Absorption of a glass I've the $n$ (refractive index of the glass sheet ) and $t$ (the thickness of the glass sheet)
with this information, how can I find the amount light absorption of the glass sheet?
 A: Possibly some semantic confusion here. Glass, with a simple refractive index, does not "absorb" light, it is transparent. Therefore the amount of light that emerges on the other side, for a given angle of incidence, is independent of the thickness of the glass.
Instead, some of the incident light is reflected from the first boundary as the light enters the glass, and some is reflected from the second boundary as it exits the glass. Perhaps this is what you are calculating? Have a look at Fresnel Equations.
Of course, real materials do absorb/scatter light, but you need a complex refractive index to sort that out. Do you have a complex refractive index? If you do then the light is exponentially attenuated as it travels through the material, but the amount of intensity attenuation depends on the (vacuum) wavelength of the light, $\lambda$, and the path length through the glass, roughly as $\exp(-4\pi \kappa x/\lambda)$, where $\kappa$ is the imaginary part of the refractive index and $x$ is the path length (which will be $t$ for normal incidence, but larger for non-normal incidence).
A: To add to Rob Jeffries's answer: the absorption data for glass are separate from the refractive index and are measured by measuring the attenuation of light through a known thickness of glass, after taking account for the reflected amounts as described in Rob's answer.
Theoretically, the refractive index and the absoption data are united in a complex propagation constant for the material, which is an analytic function of a complexified frequency in the right half plane. This means that the refractive index and absorption are actually related by the Hilbert transform, known in this context as the Kramers Kronig relationships. So one can theoretically calculate the imaginary part of the RI: the catch is that one needs to know the refractive index for all wavelengths. This means that practically, one must resort to the measurement described in my first paragraph and effectively treat the refractive index and absorption as separate data for the glass.
