Transfer of energy to a medium as a particle or electromagnetic radiation passes through it. Absorption of electromagnetic radiation is the combined result of Compton scattering, σ, and photoelectric absorption, τ. It may be quantified:
where, t = thickness, ρ = Mass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much, and μ = mass absorption coefficient, which combines Compton and photoelectric effects (μ = σ + τ).
Some or all content above used with permission from J. H. Wittke.