@mfb: Because it relates to the dimensions of the charge distribution? But as I said, I only meant it as a kind-of best case estimate. If you know any better way of guessing the cross section without the need for detailed quantum mechanical calculations, I'd be glad.
Maybe I should ask the...
When I woke up today in the morning, I had the stupid idea of trying to remember some of my knowledge from university. As it turned out, this was easier thought than done, especially given my still drowsy state of mind.
I want to roughly estimate the penetration depth of visible light in air...