Light reflection and transmission in glass
When the light meet the glass surface, some of the light is reflected. The fraction of the reflected and transmitted light is determined using Fresnel equation. The easiest format of the equation is when light perpendicular to the surface of the glass.
R=((n1-n2)/(n1+n2))^2
where R is the reflection, n1 is refractive index of the medium where the light come from; n2 is the refractive index of the glass. This case is only light reflected from the front surface of the glass. Light actually will reflect on the back (assuming the glass sample has flat and parallel surfaces). Thus, total reflection:
2R/(1+R)
Optics 