Glass refracts and depending on what it's made of, will absorb some frequencies. > For normal incidence, approximately 4 % of the light is reflected; this valu

Glass refracts and depending on what it's made of, will absorb some frequencies. > For normal incidence, approximately 4 % of the light is reflected; this value is determined by the refractive index of the glass. For most glasses with a refractive index of 1.5, reflection losses at the surface result in an approximate 4% decrease in light intensity.