Our instructor told us about this phenomenon:
The back of the glass slab is silvered, and M' is the front-shifted image of the silvered surface. While explaining reflection off of this, he stated the object distance to be; $$u = x + t/n$$ Apparently, light reflects off the image of the silvered surface, forming an image equally spaced behind M'.
How is this possible? I have heard of virtual objects, but can they serve as mirrors?