Welcome to Optical Illusions

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Welcome to Optical Illusions
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    Welcome to the fascinating world of
ILLUSIONS 
Click to continue

• Illusions are a very nice window into how the
  brain works, because illusions can reveal the
  hidden constraints of the visual system in a way
  that normal perception fails to do so.

• In addition, illusions are fun, because they
  combine both the element of joy as well as the
  element of surprise.

• We hope that you will enjoy our selection.

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Distortion Ilusions
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Are these lines parallel? Confirm with your
ruler.
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Ambiguous images
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Do you see an old man or a young one?
If you search hard you will find both.
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And here what do you see? a young woman or an
old one?
Go to next slide for hints
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The old womans eye is the young womans
ear. The old womans nose is the young womans
chin. Can you see both women?
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Underneath the yellow and brown cube, do you see
a step or a red and beige cube?
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What do you see in the left side of the picture?
Click for hints
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Did you see a woman or a hand?
Click to see image again
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Look carefully at the railroad.
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Is the following word true or false? Click
to see it.
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What do you read?
Upside Down?
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See what happens when we rotate the image to the
right.
Click to see it rotate.
The angle of rotation is only180 It seems to
have rotated 360
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This picture will be analized in the following
pictures
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     Follow the staircase around. Can you
determine the lowest or highest step?     What
happens when you go around in a clockwise
direction?     What happens when you go around
in a counter-clockwise direction? Click to see
image.
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     The is a physical model of an impossible
staircase designed by genetist Lionel Penrose.
It is the first impossible object ever made and
served as an inspiration for M.C. Eschers famous
print that incorporates this staircase,
Ascending and Descending  
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   The actual model is separated at the right
stair, but you can't see the split, because your
visual system assumes that it is seeing this
model from a non-accidental point of view hence,
it assumes that the stairs are joined.
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Paradoxes
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     Although the impossible triangle certainly
looks possible at each corner, you will begin to
notice a paradox when you view the triangle as a
whole. The beams of the triangle simultaneously
appear to recede and come toward you. Yet,
somehow, they meet in an impossible
configuration! It is difficult to conceive how
the various parts can fit together as a real
three-dimensional object.
  What's wrong with this figure?
So What's Going On?
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   Given the chance to interpret an image as
three-dimensional, your visual system will do so.
It does not generally take a perspective drawing
and reinterpret it as flat, because there is a
spatial paradox.
It is not the drawing itself that is
impossible, but only your three-dimensional
interpretation of it.
   It is possible to construct a physical
model of the impossible triangle. See the
example of the picture on your right. The true
construction is revealed in the mirror.
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    It is more important for your visual
system to adhere to these constraints than to
violate them because you have encountered
something that is paradoxical, unusual, or
inconsistent.
Your visual system, however, is very
constrainted by how it interprets two-dimensional
pictorial images into three-dimensional mental
representations.
It is with the help of such constraints that
your visual system assigns depth to each point in
an image. 
It would lead to biological disaster if you were
blind to the unusual, inconsistent, or
paradoxical.
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This can be summed up in
"The Generic View Principle"
which states that your visual system assumes
that you are viewing something from a
non-accidental point of view. This holds
unless there is information to the contrary.   
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In 1961 the Dutch graphic artist M. C. Escher
inspired by Penrose's version of the impossible
triangle incorporated it into his famous
lithograph "Waterfall."     
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Since The waterfall appeared, the
impossible triangle has reappeared in countless
versions. Because of its popularity,
many people consider the impossible triangle to
be THE impossible figure, and are astonished to
find that there are an infinite number of
impossible figures possible.
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Click to see more about this picture
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     Cover up parts of the figure. If you cover
the top half, you will find that the bottom part
of the figure is entirely possible. In this case,
you interpret the foreground figure as being
built of flat faces constituting two rectangular
prongs.       
 Now look at only the bottom half of the figure.
You interpret this figure as built of curved
surfaces constituting three separate cylindrical
lines.    
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     The two parts that are joined in this figure
have different interpretations of their shapes
when they are perceived separately. Furthermore,
when you join the two parts, surfaces that have
one interpretation (part of the foreground
figure), get a different interpretation (part of
the background). The figure therefore violates
the basic distinction between the background and
an element of the object.
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This ambiguity makes the figure violate another
basic distinction, that between flat and curved
surfaces, where a flat strip twists into a
cylindrical surface
Counting paradox. This suggests that your
visual system compares different regions by
counting. This may be one of the few figures that
reveals that your visual system also counts.
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This is a three dimensional model of the
impossible trident or disappearing column made by
Japanese artist Shigeo Fukuda in 1985. You
can see three cylindrical columns at the top and
two rectangular columns at the bottom.
Only from one
critical angle does the illusion work.
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This picture can be explained in the same way as
the trident.
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Are you looking at the Chess board from
underneath or from on top?
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Fantasy Ilusions
You see what is not there
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Did you see a cube?
It is not there. You are imagining it.
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     Steadily fixate on the black lightbulb for
thirty seconds or more. Try not to avert your
gaze. Immediately turn your gaze to the white
region on the right ajoining the bulb.
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You should have seen a glowing light bulb!So
What's Going On?      The glowing white light
bulb you see on the white screen after staring at
the black light bulb figure is called an
afterimage. When you focus on the black
light bulb, light sensitive photoreceptors (whose
job it is to convert light into electrical
activity) in your retina respond to the incoming
light.  Other neurons that receive input
from these photoreceptors respond as well. As you
continue to stare at the black light bulb your
photoreceptors become desensitized (or
fatigued).     
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  Your photo-pigment is "bleached" by this
constant stimulation.
The desensitization is strongest  for cells
viewing the brightest part of the figure, but
weaker for cells viewing the darkest part of the
figure. Then, when the screen becomes white,
the least depleted cells respond more strongly
than their neighbors, producing the brightest
part of the afterimage the glowing light bulb.
 
Most  afterimages last only a few seconds to a
minute,  since in the absence of strong
stimulation, most nerve cells quickly
readjust.    
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The two center squares reflect the same amount
of light into your eyes but, because of the
simultaneous contrast effect, they look
different. You can prove this to yourself by
covering up the areas surrounding the two center
squares.  
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    Many magicians use the principle of
simultaneous contrast to conceal parts of their
magical apparatus. For example, perhaps they
want to conceal the supports of a floating body.
The magician would make the surrounding parts
brightly illuminated, shiny metallic objects,
white cloth, etc. The parts the magician
wishes to conceal are black, in front of a black
background, usually drapery, and appear even
darker to the viewer, dazzled by the rest of the
display. The eye cannot make out the details
in the darker parts.
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The underlying mechanism behind it is not yet
fully understood.
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This slide show was prepared by
Patricia Linn
with pictures and explanations from the
following Web page
www.illusionworks.com Illusions Art Museam
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THE END