On the properties of the ‘potential’ neutron dispersion law in a refractive medium

It is known that the so-called potential law of neutron-wave dispersion in matter is distinct from all other dispersion laws in that the normal component of the wave vector in a medium depends only on its vacuum value. There is also another feature of the potential dispersion law, formulated as the statement that the neutron group velocity in a medium is equal to the vacuum velocity times the refractive index. Although generally accepted as obvious, this statement turns out to be valid only for the potential dispersion law, for which the neutron effective mass in a medium is equal to the inertial one.