Opacity

Q: What is the difference between κ ν and κ λ ?One may form a mean of κ over frequency or wavelength,e.g. a harmonic mean with dB ν (T)/dT as weighting function:1/κ R =∫ 1/κν dBν(T)/dT dv∫ dBν (T)/dT dvFor optically thick gas (like in stellar interiors) one mayfind for the total radiative flux∝1/κ R dT/dsObviously, this is analogous to the Equation of Heat Transfer with 1/κ Rplaying a role corresponding to conductivity. κ R , and similar means, areoften called ”opacity”. Sometimes the term is also used for κ ν .