The unbiquitous porod relationship, as published in many places, is often written as:

*lim I = K/s^4.*

Here, *s* is the scattering vector, described by *s=2/lambda sin(theta)* with *lambda* the radiation wavelength, and *theta* as half the scattering angle.

Strangely, though, the limit in the porod relationship is often written as the limit when *s* becomes infinity. Looking at the description of *s*, however, one sees immediately that this is not possible, whilst the maximum value of *s* (with a constant wavelength) is *2/lambda* for a scattering angle of 90 degrees.

So how am I to read that porod relationship then? as a limit where *s* goes to *2/lambda*, or where *s* goes to an unreachable infinity?

**Addendum:**

Theoretically, the scattering vector $q$ or $s$ can indeed go to infinity, but only if the radiation wavelength approaches zero. Nevertheless, it remains interesting why the limit is not to *2/lambda*, but instead is written as it goes to infinity.

hi, brian,it is interesting to talk to you here.nice blog, nice content although i can not understand everything about your PhD project. good luck!