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You can demonstrate an interesting phenomenon with
a flashlight and three flat rectangular mirrors that have
no frames. Plastic mirrorred wall tiles would be ideal. Place
two of the mirrors face down and edge to edge. Leave a space
between them a little wider than the thickness of a mirror.
Tape the two mirrors together. Stand these two mirrors up on
edge with the mirrors making a 90 degree corner. The shiney
sides should be toward each other. Lay the third mirror down,
shiney side up, against the other two as if it is the floor
of the corner. Shine the flashlight into the corner where the
three mirrors meet. Look for the reflected beam. It seems to
have disappeared.
Here's the explanation. The three sided reflector has some
peculiar properties. If a beam of light shines onto one of the
mirrors, bounces to strike a second mirror and bounces again
to strike the third mirror, then the beam is reflected back
parallel to the incoming beam from the flashlight. The distance
between the two parallel beams is determined partly by the
distance from the corner where the incoming beam strikes the first
mirror. So shining the beam close to the corner makes the
reflected beam close to the incoming beam. Shining the flashlight
exactly into the corner puts the two beams together. It then
could seem as if there is no reflected beam. But look behind
you and you will see a shadow cast by your hand holding the
flashlight.
Close one eye and look into the corner. Then do the same thing
but use the other eye. The image in the mirror is always
just that of the open eye.
A laser pointer can be used in place of the flashlight.
Clapping chalkboard erasers together puts chalk dust into the air
and makes the beams more visible.
A one meter square reflector made of glass cubes was left on
the moon to reflect a laser beam from earth. It works on the
same principle as the corner reflector described here. It is
used to accurately measure the distance between the earth and
the moon.