Nauenberg1 has pointed out some problems with my article.2 Although I agree with parts of his criticism, I cannot follow all of his arguments.

I agree that my approach of using a simple two-level system was oversimplified and that a proper application of Einstein’s derivation is not possible if only a two-level system with well-defined energies is assumed. However, I do not agree with Nauenberg1 that my explanation of how a line spectrum is transformed to the continuous spectrum of thermal radiation is invalidated.

My intent was to provide a simple radiation transfer model to explain how line spectra are transformed into continuous spectra of blackbody radiation. To follow the apparently simplest approach, I used a pure two-level system with the simplest line shape, a Lorentzian, usually related to the natural line width of the transition between two energy levels. I agree that this choice led to...

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