TITLE: Where to build a bridge to cross a river in the shape of an annulus
QUESTION [47 upvotes]: There is a river in the shape of an annulus. Outside the annulus there is town "A" and inside there is town "B". One must build a bridge towards the center of the annulus such that the path from A to B crossing the bridge is the shortest possible. Where to build the bridge?

REPLY [4 votes]: The problem is equivalent to the famous Alhazen's problem in optics, which was solved geometrically by, among others, Christiaan Huygens:
http://www2.kenyon.edu/Depts/Math/Aydin/Teach/128/AlHazen.pdf 
https://en.wikipedia.org/wiki/Alhazen%27s_problem
https://books.google.com/books?id=r-UBPLPyrOcC&pg=PA97