Here’s what happens: Not Much. Its dark, almost black, with the mirrors outline pictured.
I know the results are a little anti-climatic, but will you just leave without knowing why?
I knew you wouldn’t!
A scanner works per following mechanism:
Through the glass bed we put the document on, a beam illuminates the paper, which is the (mostly) green beam we see if the lid is open. With this light, the document is reflected on to an angled mirror inside, which is designed to move along a track at the same pace. This mirror’s reflection is captured by a fixed mirror in the scanner. The fixed mirror’s image is captured by a charge-coupled device (CCD) and saved on to your hard-drive.
Other setups exist, but this is the simplest one.
When a mirror is put on to the scanner bed, the external mirror (our mirror) reflects the inside of the scanner base, which is essentially black. That’s what the light reflects, and what the moving and fixed mirrors reflect to the CCD.
You may see a few blotches and a few finger-prints, but other than that, the result IS a little anti-climatic for the geek’s taste!