We recently reported on the
discovery of a giant eyeball on a beach in Florida and
thought it would be fun to have a look at our own eyes. In
particular, we will look at our blind spot (scotoma in scientific language) and try some
experiments that prove that our eyesight may not be as perfect as
we think.
What is a blind
spot?
A blind spot is a gap in your
vision. Normally we can see quite a lot of things at the same
time, even if some of them are a bit blurry. When you are
focussing on one thing in particular, you will still see other
things "out of the corner of your eye."
That's how you know to duck if a
ball comes out of nowhere towards your head. Your eye and
brain spot the movement at the edge of your vision and you duck by
instinct (hopefully).
So let's try an
experiment...
In this experiment we will make one
of these letters disappear. You can look at the A and B above
on the screen, or get a blank piece of paper and write the two
letters about 15 cm apart.
Now, close your left eye and with
your right eye look at the letter A. Start with your head about 50
cm (20 inches) away from the image. Making sure you are
really focussed on the letter A, move your head forward very
slowly. As you do this, the letter B will suddenly disappear
from the corner of your eye.
Blind spot experiment
number 2
This time close or cover your right
eye and look at the red dot with your left eye. Like before, from a
starting distance of about 50 cm, gradually move closer to the
image. At a certain distance it will seem like there is just one
blue line instead of two.
Your brain is filling in the gap
all by itself and you are seeing something that isn't actually
there!
There are some other experiments to
try on Professor Eric Chudler's Neuroscience for Kids website. Have a
look for yourself.
What's
happening?
So why are our eyes playing tricks
with us? First of all, it's important to understand how our eyes
work. Have a look at the BBC video below:
So you see, when the light enters
your eye it hits the back of your eyeball where nerves pick up the
signal and send it to your brain.
What about the
blindspot?
Looking at the diagram above you
can see that the optic nerve connects to the back of your
eyeball. Where it first attaches itself there is a section of
your eye that cannot detect light. This is the blind
spot (circled in red).
Normally we don't notice it, but
when we only use one of our eyes (like in the experiments above)
then it becomes much more obvious.
It's hard to imagine that we might
miss something that's right in front of our eyes, but we can!
All the more reason to really pay attention when
you are walking near or crossing a busy road!