PX282 - C9a - sources of opacity

any photon passing within an area of a particle can be absorbed or scattered in the following ways:

bound-bound absorptions

photons with the exact energy as the electron energy level gaps will be absorbed
this causes energy level transitions
PX282 - C9b - bound-bound absorption

bound-free absorptions

photons with sufficient energy will get absorbed by particles
this ionises the particles, ie. electron gets removed

h ν \to ϕ + \frac{1}{2} m_{e} v^{2}

where, $ϕ$ is the work function energy, required to free an electron
PX282 - C9c - bound-free absorption

free-free absorptions

free electrons absorb photons and gain momenta

h ν \to \frac{1}{2} m_{e} v^{2}

PX282 - C9d - free-free absorption

scattering

photons can be scattered by free or bound electrons
eg: thomson, compton, rayliegh, etc
PX282 - C9e - scattering

deionized hydrogen

PX282 - C9f - deionized hydrogen