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This equation should read $$Q=-\partial I/\partial z=\alpha I$$roam said:I also need a little help to walk me through the calculation of ##q## in your post #32.

The energy absorption in a differential distance ##dz## is:

$$dI=-\alpha I dz,$$

so using your convention for measuring z:

$$Q=\partial I/\partial z=-\alpha I.$$