Aurizon did a good job of explaining the fundamentals. If you want to get into the subject more deeply, there are any number of sources which explain so-called "surge impedance" of conductor arrangements, closely enough related to your question that the exponential need will be understood. On such source is AppCad, a downloadable collection of RF information and calculating tools originated by Hewlitt-Packard. However, it is more of a calculator than an explanatory tutorial.
Incidentally, not all RF power absorbers need be exponential. Some for frequencies less than 1 GHz are made from resistive elements short enough to not be high in SWR despite lack of taper. Instruments made by Bird Engineering fall into that category.
