This is a very simple activity that doesn't need any special equipment at all, except the circle and cross below.
+
o
What to do:
Close your left eye
Stare at the cross with your right eye.
Keep staring at the cross and move your head towards the page.
At the right distance, you should find the circle will vanish.
Keep your head still.
Close your right eye and open your left eye.
Look at the circle with your left eye.
You should find the cross disappears.
So why do the objects vanish? Because you have a blind spot in each eye.
You can see because light comes in through the pupil (the black spot in the centre of your eye) and is focussed on the inside of the back of your eye. This area is called the retina and it is covered in light sensors. These sensors detect the light and send a message to your brain. A few weeks ago (see Corner of your eye) I wrote about how different sensors are arranged around your retina. However, what I didn't mention then is that there is a hole in everyone's peripheral vision, a spot where there are no sensors at all.
For you to see something, the light sensors in your eye need to send messages to your brain. The messages travel through a series of nerve cells to your brain. The nerve cells are actually laid on top of the sensors in your eye, between them and the light. All the nerve cells in the eye lead to one point, where the nerves go through the retina and off to the brain. At the point where the nerves go through the retina, there are no light sensors. This means there is a point on each of your eyes where light cannot be detected. This is your blind spot.
When you look at the o with your left eye, if your eye is at the correct spot the light from the + strikes your blind spot, so you can't see it.
A close up of your blind spot, showing how the optic nerve goes through the retina, so there are no light sensors there.
When you have both eyes open, objects in the blind spot of one eye can normally be seen by the other eye and you don't notice your blind spot. When the signals from your eyes reach your brain, they are processed so even when you only have one eye closed, your brain edits out the blind spot so you are unaware of it.
If someone has only got one eye, they need to watch out for objects in their blind spot, especially when driving or riding a bicycle.
