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- Where does this formula of absorption cross section come from?

My professor gave us a formula for absorption cross section, but he said he did not remember where he found it.

The formula is given by

$$\sigma_a =\frac {1} {| \mathbf E_i|^2} \int_V k \epsilon^{''}_r | \mathbf {E} |^2 dV = k \epsilon^{''}_r |\frac 3 {\epsilon_r +2}|^2 V.$$

Where does this formula come from? I want to know what those variables stand for.

He gave this formula just after the formula of Rayleigh scattering cross section. It seems these two equations are related somehow, because he also gave the counterpart-formulars in Mie scattering theory.

The formula is given by

$$\sigma_a =\frac {1} {| \mathbf E_i|^2} \int_V k \epsilon^{''}_r | \mathbf {E} |^2 dV = k \epsilon^{''}_r |\frac 3 {\epsilon_r +2}|^2 V.$$

Where does this formula come from? I want to know what those variables stand for.

He gave this formula just after the formula of Rayleigh scattering cross section. It seems these two equations are related somehow, because he also gave the counterpart-formulars in Mie scattering theory.