Hi Matthew and Matt, Thanks for your rapid replies and explanation describing the reason for the difference between the rocking curve and the Darwin. I indeed only have little bit of idea about DuMond diagrams and not much and would have to seek further help to understand it better other than that it is a transfer function and its application in a symmetric Bragg reflection. In the meantime, the figure attached as PDF file and caption given in quotations is what I am trying to understand with the help of the text in the book in addition to what Matthew explained. I hope that over the weekend I am still confused but at a higher level. Thanks once again. Regards, Sankaran Start quote" Figure Caption: Non-dispersive geometry (left): X-rays from a white source are incident on two crystals aligned in the same orientation. The central ray (full line) will be Bragg reflected by both the crystals and will emerge parallel to the original ray. A ray incident at a higher angle than that of the central ray will only be Bragg reflected if it has a longer wavelength. The angle of incidence this ray makes with the second crystal is the same as that it made with the first, and will be Bragg reflected. The DuMond diagram in the lower part shows that a scan of the second crystal has a width equal to the convolution of the Darwin widths of the two crystals, independent of the incident angular divergence. Dispersive geometry (right): A ray incident at a higher angle than the central ray at the first crystal will be incident at a lower angle at the second crystal . The second crystal must be rotated by the amount 2(DELTAtheta)in for Bragg's law to be fulfilled. " End quote. Jens Als-Nielsen -- Universität Stuttgart Institute of Physical Chemistry Pfaffenwaldring 55 70569 Stuttgart Tel.: +49 711/685-64463 Fax: +49 711/685-64443