By giving a lecture, providing lecture materials or producing lecture notes you are effectively publishing material.
This material, just like a more formal book, is subject to standard copyright conditions.
As such when reproducing material from other copyrighted sources suitable attribution must be given and permission first received as necessary.
Doing so not only ensures compliance with the law it set a good example to the students who in turn are expected to follow such guidance when producing there own reports, dissertations and theses etc.
Also see the Copyright Statement for the departmental website.