Note that the effective cross sectional area of a particle is not really due to a physical size, but instead can be thought of as a propensity, or likelihood, for an atom to interact with a photon -- the more likely the interaction is, the bigger the cros

Now, for gas with lots of particles, it makes sense to ask not about the cross section of one particle, but the cross section per unit mass, k (cm2/g), which is called the absorption coefficient, or opacity

Substituting for s in the last equation gives the radiation force per unit mass, or the radiative acceleration

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