Travel information on how to reach the the Physikzentrum Bad Honnef can be found here.
There is a fast ICE-train from Frankfurt Fernbahnhof to Siegburg/Bonn, which
goes nearly every hour. From Siegburg/Bonn train station go to the tram station
and take the tram 66 to Bad Honnef, which takes 1.11 hours. Alternatively, you
can take a Regional-Express from Siegburg/Bonn to Troisdorf, change there to
the train to Bad Honnef (approximately 40 min.).
You can find more information on the DB Bahn page.