The Pulfrich
Illusion Science Lab Geocache!
If there are
any white instruction sheets left, feel free to take an extra copy.
Please leave the colored master however. Also leave the glasses and
the pendulum. Others will need them to see the illusion on
location. Park at 42 28.181 by 92 24.183. By the way, the
background picture is of my son; he goes by
Lycan.
This science lab
geocache should give most people an opportunity to experience the
strange perceptual optical phenomenon called the “Pulfrich
Illusion.” The working parts of this cache are a pair of sunglasses
from which one lens was removed and a pendulum on a hook. The
absent lens I made into the “The Pulfrich Illusion TB!” that
started in this cache. My wish for the TB is to travel through
English speaking countries to show them the illusion as well. [The
instructions in the TB container are in
English.]
To see the
illusion do this: hang the pendulum on a branch or something. Pull
the pendulum back only one to two feet and let it go. [The illusion
works best when the object is swinging fairly slowly.] Make sure
the ball is swinging in a straight line. Now step back about six
feet and note that the pendulum is still moving in a straight line.
Don the one-eyed sunglasses and carefully observe the pendulum's
movement. It should now appear to be moving in a distinct
elliptical pattern, not in the straight line that you saw it
move in before! The drawing pasted here shows what the illusion
will look like.
In my science
classrooms every year I get out several sets of one-eyed sunglasses
to allow the students to experience this weird effect. There I hang
a pendulum from a ceiling then start it swinging perpendicular to
the audience. To make it REALLY illusionary I stand close to the
pendulum. As it swings toward the darkened eye it appears to be
moving away from me (closer to them) BUT when it moves away from
the darkened eye it appears to be moving toward and even THROUGH
me. That makes me look like a ghost! Try this at home by holding
any pair of sunglasses vertically so that you darken only one
eye.
In class, to prove
that the illusion is real I hold up a meter stick and move my
finger straight back and forth a few inches behind it. I ask them,
“Am I moving my finger in a straight line or in an ellipse?” They
always say, “In an ellipse.” Then I begin to drag my finger across
the back of the ruler to make a scraping sound and I ask them
again, “Am I moving my finger in a straight line or in an ellipse?”
They look puzzled and smile, but again say, “In an ellipse!” As it
was with the pendulum we me standing close to it, when moving in
one direction two parts of my finger appear to be in front of the
stick, an impossibility!!
Long ago when I
read about this phenomenon in the wonderful book called “Eye and
Brain, the Psychology of Seeing” I wondered if things moving across
a TV screen would appear to be in front of the screen when moving
one way and behind the screen when moving the other way. Sure
enough, it worked! That makes for an interesting football game. LOL
Not more than a year after I made this discovery the Pulfrich
illusion was incorporated into the half time of a Super Bowl game.
In preparation for the big event people could get the required
special glasses at Quick Trip for free. The advertisement showed
true 3D depth by continuously circling around any objects that were
being featured at the time.
The way the
illusion works is this. To see the third dimension each eye needs
to see an object from a different viewpoint, just as a person’s two
eyes do normally. The greater the difference in what we call
perspective that is evident in an object, the closer it must be, or
so our brains have learned to tell us. Objects that are very close
are perceived quite differently by each eye. They are also shifted
significantly to the right or the left with respect to the
background; another depth cue for the brain. Objects that are too
far off to see in 3D appear the same to each eye and are not
shifted noticeably. So how does darkening one eye cause the
perception of 3D? The trick is this: the signal processing by the
darkened eye is slowed down. When it sends a signal to your brain
of the image it saw, the moving object was in a slightly different
place for that eye than for the other one. The brain quickly merges
the two images, one of which was “taken out of time." Because of
the delay, the dark eye saw the object from a slightly different
angle (perspective) than the other eye. Two different left and
right perspectives provide the brain with information that we have
learned to perceive as relative distance. The problem is, this time
the difference in the two images was not correct for the true
distance of the object! The degree to which the differences in
perspective is wrong is most pronounced at the points where the
moving object is going the fastest across your field of view; hence
the elliptical pattern.
If you take a home
video looking straight out a side window while moving down the
road. you can see good depth in the moving objects. While looking
at the video, if you were driving toward the left of where you are
pointing the camera, use one side of a sunglasses to darken the
right eye or the other eye if you are going the other direction.
You should see the things that are closer look
closer.
One more thing
bizarre thing you can do with the one-eye-dark trick is to turn the
TV to an all-snow station, like channel 1. Notice with both eyes
bright there is no apparent direction to the movement of the snow.
Now darken one eye and stare at the screen. You should soon see the
snow appear to be moving in an ellipse! It’s weird!! Darken the
other eye and it’ll reverse direction! I can do all these things to
some degree by simply rolling up one index finger to produce a
small hole to look through. That then is the darkener for one eye.
Even squinting one eye works a little!
The Pulfrich
Illusion is demonstrated online here.
I hope you enjoy this science lab
cache.