Light coming from
a real object cannot be distinguished from light coming from an image.
Two people sit on opposite sides of a one-way" mirror. This mirror reflects
1/3 of the light lets 1/3 through to the other side and absorbes the
other 1/3. By changing the illumination on either person one is able
to combine the two faces into one face with some features of one person
and some features of the other person. If each person presses a button
only one half of each is lit and you see a "split personality."