This Week's Experiment - #240 Waterfall Effect
This week's experiment is one that I have wanted to share with you for a long
time. The problem has been finding a way for you to see it. To really get
the biggest impact, you need a large waterfall. Realizing that most of you
do not have one of these in your back yard, I have been searching for other
ways for you to see this wonderful optical effect. You will need:
One of the following:
a waterfall
a television
access to websites
If you happen to have a waterfall nearby, you are all set. You need to be dow
nstream of the waterfall, looking up at it. Stare at the falling water for
about 30 seconds. Then look at some nearby trees or a building. What you
see should impress you. A vertical strip of the trees will seem to flow
upwards! I first saw this at Fall Creek Falls in Tennessee. At the time, I
had never heard of the waterfall effect and it gave me quite a challenge
chasing down an amazing mystery in those days long before the ease of
Internet research.
If you don't have a waterfall, you can also use your television. Many of the
news channels now have scrolling bands of text at the bottom of the screen.
As the words move across the screen, they are a reasonable substitute for the
waterfall. Stare at a point in the middle of this band of moving words for
about 30 seconds and then look at a picture or something with a nice pattern.
You will see a horizontal band of the pattern seem to flow sideways. Not
quite as impressive as a waterfall, but still pretty neat.
If you have access to websites, you are in for a wonderful treat. There
are quite a few websites which have rotating spirals which use the waterfall
effect to produce some spectacular illusions. One of the sites is:
dogfeathers.com
After staring at the spiral for about 15 seconds, look at a picture of
something with a pattern. I found woodgrain on the panels of my wall did
very well. For a special treat, stare at the spiral and then look at the
back of your hand! Be prepared to be startled.
Why do you see this? The reason is related to the color reversals that we
saw in last week's experiment. In looking at the strangely colored flag, you
"fatigued" the color receptors in your eyes. Staring at something that is
yellow desensitizes the cells in your eye to yellow. When you then look at
something that is white (a mix of all the colors), the signals that report
the yellow part of the light are weaker than the others. This causes the
white paper to look blue, as you see the spectrum with the yellow taken out.
A similar thing is happening as you stare at the motion. The big difference
here is that the "fatigue" happens in your brain, not in your eyes. The part
of your brain that deciphers the impulses from your eyes into sight also
detects motion. These "motion detectors" are direction specific. If the
signal from the upwards motion detectors are stronger, your brain recognizes
it as upwards motion. If the right motion sensors are stronger, you see
movement to the right. If all of the signals are the same strength, you
don't see any motion.
If one part of your vision continually sees motion downwards, while the rest
of your vision sees stationary objects, the part that sees the motion becomes
fatigued. As the nerves fire continuously, the signal weakens. If you then
look at a stationary pattern, the signals would normally return to all the
same strength, but instead, the downwards sensors are tired and their signal
is weaker. The normal signal from the upwards sensors is stronger and tells
your brain that you are seeing things move upwards.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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