I placed a few books on a mirror, then placed a
small item on top of the middle of the books. Why does the mirror
reflect that small item and not be blocked by the much larger books?
I've always been fascinated how a whole room can be reflected in a small
The entire space on the reflecting side of a flat mirror can be seen in the
mirror if the observer looks at the mirror at different angle (i.e., moves
her head about).
For a given position of your eyes with respect to the mirror, you can see
the portion of the space (on the reflecting side of mirror) that falls
within the angle that you eyes make with the sides of the mirror. This is
easier to illustrate graphically, but lets do a thought experiment to
illustrate the point. If you take a flash light that produces a diverging
(expanding) light beam and point it at a mirror (illuminating the entire
mirror surface) in a dark room, you would see that the reflected beam from
the mirror illuminate part of the room on the reflecting side of the
mirror. This part of the room is the same part that you eye (placed at the
same location as the flashlight) would see if you turn the lights on in the
room and look into the mirror.
Regarding the small object on the books, you can see it if you properly
position your eyes such that the angle that your eyes make with the mirror
when reflected and traced out into space would have the small object in its
field. (As you know, then angle of incident and reflection are the same
off a mirror, a property that you would use in tracing out the said angles
Hope this answers your question.
Dr. Ali Khounsary
Advanced Photon Source
Argonne National Laboratory
If I were to attempt the experiment described (place a mirror parallel to
the floor, place a book on it, then place a small object on top of the book) I
would not expect to be able to see the object in the mirror unless the object
was tall enough to be seen sticking up above the book.
You will see in the mirror those parts of the object that would be visible
if you replaced the mirror with a sheet of glass and then adjusted your
position so your eyes were the same distance below the glass as they were
above the mirror.
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Update: June 2012