On the portions of the original document which are blank the light bounces off the white surface and is reflected to mirror 1, then to mirror 2.
(the original document; are; blank) (the light; bounces; L:On the portions of the original document) (the light; is reflected then; to mirror 1)
the original document the light the light
are bounces is reflected then
blank
L:On the portions of the original document
to mirror 1