In an SLR, the light bouncing off the mirror goes through a pentamirror or pentaprism before being reflected to the viewfinder. However, it seems to me that it would be possible to project an image to the viewfinder simply by using two mirrors, the upper one at a complementary angle to the lower one, much like a simple periscope.
Such a system might have benefits in terms of light transmission, as the light is only reflecting off 2 surfaces instead of 5, leading to a brighter viewfinder. This is such an obvious thing that I feel I must have misunderstood the purpose of the pentamirror/pentaprism.
Can anyone elaborate on why exactly such a complex path of reflecting surfaces is needed in an SLR?