Summary: | The application of perturbation theory to a three-layer weakly-guiding slab waveguide composed of lossy dielectric media yields a simple formula for the attenuation coefficient alpha of a guided mode: α = (Σ<sup>3</sup><sub>i=1</sub> α<sub>i</sub> P<sub>i</sub>) / (Σ<sup>3</sup><sub>i=1</sub> P<sub>i</sub>), where α<sub>i</sub>, P<sub>i</sub> are respectively the loss coefficient and model power in region i (i = 1,2,3). It is shown that this result can also be obtained from arguments based purely on geometric optics. The result is easily extended to apply to circularly-symmetric optical fibres where it yields confirmation of an earlier approximation for the power ratios P<sub>i</sub>/Σ<sup>2</sup><sub>i=1 </sub>P<sub>i </sub>.<sub> </sub>
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