International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. B, ch. 5.1, pp. 543544
Section 5.1.6.4. Reflecting power^{a}Laboratoire de MinéralogieCristallographie, Université P. et M. Curie, 4 Place Jussieu, F75252 Paris CEDEX 05, France 
For an absorbing crystal, the intensities of the reflected and refracted waves are where and is given by equation (5.1.6.1).
Depending on the absorption coefficient, the cosine terms are more or less important relative to the hyperbolic cosine term and the oscillations due to Pendellösung have more or less contrast.
For a nonabsorbing crystal, these expressions reduce to
What is actually measured in a counter receiving the reflected or the refracted beam is the reflecting power, namely the ratio of the energy of the reflected or refracted beam on the one hand and the energy of the incident beam on the other. The energy of a beam is obtained by multiplying its intensity by its cross section. If l is the width of the trace of the beam on the crystal surface, the cross sections of the incident (or refracted) and reflected beams are proportional to (Fig. 5.1.6.5) and , respectively.
The reflecting powers are therefore: Using (5.1.6.6), it is easy to check that in the nonabsorbing case; that is, that conservation of energy is satisfied. Equations (5.1.6.6) show that there is a periodic exchange of energy between the refracted and the reflected waves as the beam penetrates the crystal; this is why Ewald introduced the expression Pendellösung.
The oscillations in the rocking curve were first observed by LefeldSosnowska & Malgrange (1968, 1969). Their periodicity can be used for accurate measurements of the form factor [see, for instance, Bonse & Teworte (1980)]. Fig. 5.1.6.6 shows the shape of the rocking curve for various values of .

Theoretical rocking curves in the transmission case for nonabsorbing crystals and for various values of : (a) ; (b) ; (c) ; (d) . 
The width at halfheight of the rocking curve, averaged over the Pendellösung oscillations, corresponds in the nonabsorbing case to , that is, to , where δ is given by (5.1.3.6).
References
Bonse, U. & Teworte, R. (1980). Measurement of Xray scattering factors of Si from the fine structure of Laue case rocking curves. J. Appl. Cryst. 13, 410–416.Google ScholarLefeldSosnowska, M. & Malgrange, C. (1968). Observation of oscillations in rocking curves of the Laue reflected and refracted beams from thin Si single crystals. Phys. Status Solidi, 30, K23–K25.Google Scholar
LefeldSosnowska, M. & Malgrange, C. (1969). Experimental evidence of planewave rocking curve oscillations. Phys. Status Solidi, 34, 635–647.Google Scholar