Everything is not what it seems…
The
arrows and the arrow heads
A simple optical illusion.
This is a simple illusion that can be used as the basis for
a science project. The two horizontal lines that you see here are the same
length, but one appears longer than the other.
If you set up one
of these as a static drawing, the second arrow can be made as a slide, so you
can get people to try to match the two lines.
You can get people
to slide the lower arrow in and out until they feel that the two lines are of
equal length. Generally, they will push the lower arrow in until the lower line
is shorter: record the values for each subject, and maybe a few other variables
like their gender and their age, and calculate a mean and standard deviation
for each sub-group
Notes:
It may also be worthwhile asking whether people have seen this illusion before or not. You might find it worthwhile seeing what effect it has if you change the angle of the arrow head, keeping everything else constant.
To make the measurement easier, if set the test apparatus up on an upright board, and observe from behind it as people arrange the arrow, sliding it along a slot. Then you can put a scale of some sort on the back, so you can read the length off without having to get out a ruler or measuring tape.
Most importantly, if you are going to try this sort of thing, you will need some expert advice on how to do the statistical analysis. You will usually find differences in the means and standard deviations for groups, but you need to apply tests of significance, to see if the differences are likely to be just chance variations or not. Find an expert!
As a basis for a science project, you could ask people to
say how many prongs there are, and see how long it takes them to realise that
there is a contradiction.
I once passed this drawing over to the
workshop of the lab where I was working, explaining that we wanted one of these
in steel, exactly as shown, for a seed planting experiment. They sent the
drawing back, saying there was a delay in getting the left-handed hammers they
needed.
For this book, I
added some extra shading to make the “two” side look more convincing: what
effect would it have if you took that shading away? The other factor that could
affect this illusion is its length. Maybe you could study how long it takes
people to notice something is wrong with different lengths of the same
illusion? As in all tests of this sort, it is important to get some good advice
on statistical significance, and how you measure statistical significance.
How
straight is that?
The two horizontal lines are completely straight, as you will see if you redraw this diagram for yourself, yet they look to be curved.
I have been playing with the idea of one of these, made with
black wire, where people can bend the thicker horizontal wires until the
“lines” look straight, but I have yet to work out how to measure the bending
easily.
Maybe the answer
is to have a wheel with a dial and a thread that pulls on the wire? It’s your
project…
Another way: use the index!
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