The essential physical point of the previous discussion can be
extended to the case of relativistic field theories and to different
form factors. The 
-dependence of 
is again related to
the hadron structure, and mean squared-radii can be defined in
anology with eq.(10), i.e.,
A simple example on the relation between these charge radii and the
quark substructure in hadrons is clearly offered by the low-mass
pseudoscalar mesons consisting of a spinless, s-wave 
pair.
The squared charge radius of charged pions is
known to be about 
and can be attributed to the
distribution of the (positive) u and 
constituents of a 
around their common CM.
In the 
case, similarly,
the heavier 
substitutes 
, thus reducing the 
dimensions as corroborated by the data (see below).
Finally, the negatively charged
and light d-quark in a 
is worst localized than its
positively charged and heavier partner 
giving therefore rise
to the negative 
charge radius experimentally measured.