There is perhaps no
better place for a visual illusions discussion to start than
with the visual perceptions in our normal, everyday lives. Our
daily experience reveals how ordinary changes to our sight changes
our perceptions of unchanging things. Many of these optical and
mental tricks barely raise an eyebrow as they are commonplace
and we know how they work. Not all of the following examples
may qualify as illusions, but all reveal the variations in how
* To read at night, you turn on the light. Though the printed
words are identical in dark or light, humans can only read them
in light. Without light the words disappear from human sight.
* If you enter your home and someone is sitting in a chair
in your darkened living room, you may see the form of the person
but must turn on the light or ask to find out who it is.
* We often can't identify a bird or other small animal until
we pick up the binoculars. It is with the distortion (magnification)
of our eyesight that we learn the animal's identity. Without
the distortion we can only guess.
* If there is enough sunlight glare on a downtown store window,
you can't see what is past the window while standing outside.
You can only guess what if anything is inside. Perhaps the store
is filled with people and products. Perhaps the store went out
of business last week and the inside is bare. When you walk up
to the window and shade your eyes, the glare is removed and you
see what is inside.
* Most shoppers have experienced where the color of the dress
or paint or wallpaper or couch looked different when they got
it home, caused by the difference in lighting between home and
store. Some have thought they accidentally brought home the wrong
item. Some have complained that the store intentionally used
* At night you are watching television and see something moving
outside, only to discover it is a reflection of you in the window.
You move your arms and head side to side to confirm it isn't
a scary monster.
From these common situations you get a glimpse at how knowledge
and perception are affected by even little things like light
intensity and viewing angle. Even a slight change to viewing
angle can be the difference between knowing and having to guess.
(c) david rudd cycleback, cycleback.com
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