In X-ray diffraction, what is the virtual source?

[weeman203]

In X-ray diffraction, during the experiment a X-ray beam will create an incident and diffracted beam, but what exactly is the virtual source? I may be mistaken but isn't the source the incident beam, in the attached image, it seems to suggest otherwise?

 

[Andrew Mason]

I expect that they are borrowing the concept from optics. The virtual source is the apparent source ie. An xray source. located where the xrays appear to be coming from if one extended the diffracted beam back . The incident beam comes from the real source.

AM