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Scenario A: Illusions
1. Blind Spot: ask your students to take a blank sheet
of paper (provide some scratch paper for those in need), and
place their open hand in the middle of the sheet. Using a pen
or pencil, each student is to make a small dark cross
on the paper next to the right side of his/her hand. Then
draw a dime-sized dot on the paper next to the left
side of the hand. Remove the hand, and the dot should now be
about 12 cm (5 inches) from the cross.
Ask each student to hold his/her left hand over the left
eye, hold the paper at arm's length (about 60 cm, or 2 feet)
with the other hand, and stare at the dot on the left
with he right eye only. Both the dot and the cross should
be visible. Now ask them to slowly bring the paper closer, while
continuing to stare at the dot. Ask if they notice something
strange happening.
Most students will be startled that the cross seems to disappear
when the paper is a little more than a foot away. If they continue
to bring the paper closer to the face, the cross will reappear.
Have them repeat these movements to see that there is a small
distance range when they can't see the cross. They can also do
this with the right eye covered, the left eye open, and staring
at the cross. In this case, the dot will not be seen at
about the same distance range as with the other eye. For an internet
version of this "blind-spot" test, along with an explanation,
click here. For an even better explanation,
with a great diagram (along with variations), click HERE.
This nicely demonstrates that we all have a blind spot in
each eye. These are the spots where the retinal nerves all come
together and exit the eyeball through the retina to form
the optic nerve. Our eyes are not perfect, and the sense
that we normally see a complete field of view with no blind spots
is an illusion.
2. Other illusions in nature:
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a. Ask where the sun was earlier this morning,
and where it will be later this afternoon. Next ask how it got
from the morning point to the afternoon point. Most people will
say that the sun moved across the sky. You can say "are
you sure...is that your final answer?" Eventually someone
will point out that actually that's only an illusion; in fact,
the sun stays put (in our solar system), and the earth rotates
with us on it, so that the sun only seems to move across
the sky.
b. Ask what the size of a full moon is just above the
horizon, compared to its size straight overhead. Most will say
that it is larger. In fact, they are the same size...it's only
an illusion. (Have anyone who doubts you just set up a device
at one end of a yardstick which can hold a piece of paper on
which you can mark the apparent diameter of the moon while looking
at it with your eye at the opposite end of the stick. do this
when the full moon rises on the horizon, then later in the evening
when it is overhead. Be sure to mark the apparent diameters on
the card both times. If done carefully, they will be the same.)
Somehow, the bright clarity of the moon seen close to nearby
objects (trees, houses, telephone poles, etc.) in a horizontal
direction seems larger than when viewed higher in the sky. It's
an illusion!
c. A very subtle biological illusion is the sense that
species don't change: that dogs have puppies, cats have kittens,
and people have babies. It seems that species don't produce new
species. However, this must be an illusion, since we have so
much compelling evidence showing species arising from other species,
and that even whole groups of organisms must have descended from
other groups, and this over very long periods of time, i.e. evolution.
We clearly see from the fossil record that new groups of organisms
have continuously emerged over vast 100s of millions of years.
In contrast, there is absolutely NO evidence or suggestion that
all life forms appeared instantly or magically, over, say, six
days, six years, or even 6000 years. We also have considerable
evidence of species changing, sometimes quickly, but usually
over long periods of time. In addition, we are beginning to have
a fairly good understanding of HOW such changes occur, and HOW
such changes can accumulate to produce new species and eventually
entirely new groups of life forms.
d. Ask your students to list as many other illusions
in nature as they can. (They might benefit by working initially
in small groups to do this for about 5 minutes, then share their
suggestions with the class). There are many such illusions ...just
look around, and use them as examples. The sense that the earth
is flat is a very powerful one. Planet motions and
the actual star locations in what we see as constellations
are two other areas of natural illusions.
Here are some other natural illusions, many associated in
the past (and some still) with beliefs in the supernatural:
We are in the center of the universe
Earth seems closer to sun in the summer (actually closest in
our northern hemisphere winter)
Light travels instantaneously
Lightning
Rainbows
Earthquakes
Eclipses (of sun or moon)
Disease, birth defects
Floods, hurricanes, tornadoes, rain
Fire
Stars in the dome of the sky
Everyone and every solid thing is solid matter (actually, studies
tell us that every atom is mostly empty space, and everything
material is made of atoms)
Flying saucers
Mirages
Earth stands still (even though we now know it spins at about
1000mph at the equator, races around the sun once a year (at
over 66,700 mph), travels around our galactic center, AND is
coursing through space with our galaxy.)
e. What's the point of all this? See
item #4 below.
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3. Intentional illusions:
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a. Prepare some optical illusions on overhead
transparencies. Many are available in books of optical illusions,
and even on the internet (Illusionworks). One of these
is the Ouchi Illusion; it looks great on an
overhead or in a PowerPoint display. A very clever one we offer
here requires two identical transparencies, each with two tables drawn in
perspective. Start with the two sheets in register, so that the
identical table images are on top of each other. Ask which table
is longer (it seems obvious), then slowly shift and turn one
sheet about 90 degrees, dramatically pretending to str-r-r-r-etch
the plastic, until the top of one table is directly aligned
with the other table-top, and everyone sees that they are exactly
the same size and shape! Check out a variety of other optical
illusions available. Another nice collection of optical
illusions (including the "blind spot" illusion) can
be found at Vision Education Activities.
b. If you can, do some simple sleight of hand (stage magic).
There are some very easy coin tricks you can learn, or get some
packaged tricks at a magic shop. Look for large, easy-to-see
tricks, and those not too difficult for you to learn. See "Some
Classroom Magic Resources" in box below.
c. Don't show all your illusions here; save some for
later. They are nice to sprinkle at odd moments during your course,
especially if they are even remotely relevant (such as a few
dice or card tricks in your genetics unit, when you deal with
probability). You will be limited only by your imagination and
sense of fun.
d. Also, NEVER reveal the secrets of your "magic".
This is a fundamental code of magicians. If you are ever pressed
by your students to "show how you did it", point out
that in science, we NEVER really KNOW the secrets of nature,
thereby reinforcing the fact that scientific solutions are always
UNCERTAIN.
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4. "Perception is not always reality" (with
apologies to Mercedes Benz). At some point, make it clear that
the natural world is full of illusions, and simple common sense
doesn't always work to explain such illusions. This is where
science is an especially reliable tool, able to cut through
illusions, useful to help us dig out the real story....the most
accurate explanation, the reality behind the illusions. Nevertheless,
science is not perfect; science can be fooled, too. For that
reason, people who practice the art of creating illusions professionally
("magicians" and "sleight of hand artists")
are sometimes even better equipped to see through illusory phenomena,
especially if they are intentional, as perpetrated by hoaxers
and con artists. Randi the Magician is one such professional
who has exposed such people where scientists had been fooled.
Explore the
world of Randi by visiting his web site. He is occasionally
featured on television demonstrating his success at unmasking
the tricks of those who would profit at your expense: psychic
readers, magical healers, spoon benders, clock starters, and
the like. If possible, tape one of his shows, and share with
your classes. Great for discussion.
You might find it useful to make a large banner (or have a
student do it) for your room: "Perception is not always
reality!" (Some may recognize it as the Mercedes Benz
slogan, where it has a totally (?) different meaning). In any
case, it will be amazing how often during the year you can glance
up at the banner, conspicuously and knowingly, when an appropriate
example comes up. Even some of your students will take the initiative
at times. Great! Encourage that!
5. There is a close tie between natural illusions and making
false assumptions. Actually, natural illusions exist largely
because we make certain false assumptions. For a most useful
ongoing activity, take a look at our lesson on "False
Assumptions".
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