An em wave carries a linear momentum with it. The linear momentum carried by a portion of wave having energy U is given by \(p = \frac Uc\).Thus, if the wave incident on a material surface is completely absorbed, it delivers energy U and momentum \(p = \frac Uc\) to the surface. If the wave is totally reflected, the momentum delivered is \(p = \frac {2U}c\) because the momentum of the wave changes from p to -p. Therefore, it follows that an em waves incident on a surface exert a force and hence a pressure on the surface.