This Week's Experiment - #260 Vanishing
This week's experiment is one that you have to try to believe. I know that
some of you feel that you get enough science just by reading these, but this
one you must try. You will need:
a hand held mirror
2 blank walls or two large sheets of white poster board
a friend
2 chairs
You need a place where you can stand facing one section of blank, white wall
and have another section of blank, white wall to your right. Place one chair
with its back against the wall in front of you. Have your friend sit in that
chair. Place the other chair in front of your friend, being sure that you
have a section of blank wall on your right. Have a seat in the chair.
Once you and your friend are facing each other, hold the mirror in your left
hand. Bring it up to your face against the right side of your nose. Turn
the mirror at an angle so that when you look straight ahead, your right eye
sees only the white wall beside you. Your left eye should still be able to
see your friend.
Now for the fun part. Both of you should sit very still to make this work
well. Hold your right hand up so that you can see its reflection in the
mirror with your right eye. Looking straight ahead with both eyes, you
should be able to see your friend and your hand. Now move your right hand as
if you were using a chalk eraser to wipe your friend's face away. As your
hand moves over their face, it should vanish!
What is going on? Has your friend really vanished? No, of course not. We
have just played a trick on your brain. Because your eyes are separated a
bit, they each see things from slightly different angles. Your brain has the
job of taking these two images and combining them into one. Usually, the two
images are very similar, but here, they are very different. You start with
one image of your friend and one image off a white wall. The choice for your
brain is easy. Your friend is much more interesting than a blank wall. (If
your brain thinks that the wall is more interesting, be kind and don't tell
your friend.)
At this point, your brain is focusing on the image of your friend and
ignoring the white wall. Suddenly, you start to move your right hand. Our
brains are very sensitive to movement, so it quickly focuses on the moving
hand. Here is the fun part. Since your brain is used to combining the
images from both eyes into one, it keeps the interesting part of each. Where
there has not been any movement, your brain keeps seeing your friend. In the
places where your hand has moved, it switches and uses the image of your
hand, and the wall where your hand has moved. This gives the illusion of
erasing your friend's face.
For some people, certain parts of the face will linger, even after you have
erased all the rest. The mouth and eyes are the most common parts to remain,
but it varies from person to person.
Don't worry if it takes a time or two to get the hang of this. If you can't
get it to work, try reversing things, so that the wall is on your left,
instead of your right. If it still won't work, maybe you need to find a more
interesting friend.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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