So, if matter (an atom) absorbs light (a photon) momentum is transferred to the matter
Light can force atoms to move!
r e, the radius of an electron, giving a cross section, s T (cm 2)
The flux of light, F (ergs s-1 cm-2)
The rate at which momentum is absorbed by the electron
By replacing the cross section of a single electron with the opacity (cm 2 g-1), the combined cross section of a gram of plasma, we get the acceleration due to radiation