When light meets a glass surface, some of the light is reflected, depending on the angle of incidence and the refractive indices of the glass and the medium the light is coming from (e.g., air). The fraction of the reflected and transmitted light can be determined using the Fresnel equations. In case the angle of incidence is perpendicular to the glass surface, the fraction of reflected light is given by the reflection coefficient or reflectance, R, with n_o being the refractive index of the medium the light is coming from (e.g., air with n ≈ 1) and n_G being the refractive index of the glass:

The light passing through glass is not only reflected on the front surface, but also on the back. In fact, the light may be reflected back and forth several times. Therefore, the total reflectance through a glass window is 2·R / (1+R). According to these formulas, about 8% of the light is reflected from common soda-lime window glass, assuming perpendicular incidence angle and no absorption and scattering.

If absorption and scattering are neglected, then the transmission coefficient or transmittance, T, is always 1-R.

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