multipath reflection fresnel zone

Hello,

In http://adt.library.qut.edu.au/adt-qu...ic/02whole.pdf
at pp.49-53 the author talks about fresnel zones. Especially the fresnel zone due to reflections interest me. The author gives 2 equations 2.24 and 2.25 which are a bit foggy to me. These equations quantify the radius and the semi-major axis of the first fresnel zone on a reflective surface. The problem is the exact definition of these quantities with respect to figure 2.52.

Thanks.
Best regards.

The equation 2.24 is the radius of the FIRST fresnel zone. In the
fig. 2.52 is the dark circle marked with n=1(where the point
M is). In other words, is the distance from the center of
the circle to the point M.