Cross Section
We keep finding quantities that look like L2 in the numerators of these definitions, so physicists like to describe these as cross-sections, i.e. we think about the effective cross-sectional area of the electron as it interacts with the photon.
t/r µ E-3 of the photon (most elements),
t/r µ E-2.96 of the photon (for lead)
But energy = hc/l, so for most elements t/r µ l3