On the difference in the depths of formation of the centers of weak Fraunhofer lines and the far wings of strong ones

The Voigt function H(a,v) influences the absorption coefficient of the centre and of the wing of a Fraunhofer line differently. The absorption coefficient of the line centre is K(0) ≡ N/Δλ_D, and the one of the far wings is K(v) ≡ Nγ. Here N is the concentration of absorbing atoms, Δλ_D is the Doppler width, γ is the damping-broadening constant, sharply increasing downward into the photosphere. Due to this, the effective depths of formation of far wings of strong lines are larger than those of weak lines having the same atomic parameters.