Question

The amount of light reflected

From the front surface of a common windowpane is about

Answer 1

The amount of light reflected from the front surface of a common windowpane is 4% it is because all the light will refracted and also loss.

The loss of energy is due to a transfer of momentum from the photon to the surface. It is not really the same as say a ball losing energy when it bounces, there are several contributing factors to why a ball loses energy and only one for light.

If a photon has p = E/c when it reflects it has -p (same magnitude, opposite direction). As a result there is a deficit of 2p which is transferred to the reflective surface, this increases the mirrors kinetic energy and as such must decrease the energy of the reflecting photon.

The fractional change in energy is very small due to the fact that the mirrors mass is much larger than the energy of the photon. I can do the math if you wish but it is roughly a factor of E/mc2 .