This is a hands-on demonstration.
Work Step by Step
With the 90-degree angle between the mirrors, the image straight ahead of you is not reversed. With the 60-degree mirrors, the image straight ahead of you is reversed. Try it and see! This page contains some additional information, as well as a ray diagram of the 60-degree case: http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/80.09.html