Seeing...
your Blind Spot!

Popular Science to Try at Home

Did you know your retina (the back part of your eye) reacts to light and sends signals to the brain – which is what seeing is. There is, however, part of the retina that doesn't give visual information. This is your eye's blind spot.

You are so used to it, you don’t know it is there – until you try this simple experiment!

What you will need

One 3 x 5 inch (8 x 13 cm) card or other stiff paper
A ruler

How to do it

Mark a dot and a cross on a card as shown.
Hold the card at eye level at arm's length, with the cross on the right.
Close your right eye and look at the cross with your left. Notice that you can also see the dot.
Focus on the cross, and slowly bring the card toward your face. The dot will disappear, and then reappear, as you bring the card closer.
Now close your left eye and look at the dot with your right. This time the cross will disappear and reappear as you bring the card slowly toward your face.

What’s happening?

The optic nerve carries messages from your eye to your brain. This bundle of nerve fibres passes through one spot on the retina, of your eye. In this spot, there are no light receptors. When you hold the card so that the light from the dot falls on this spot, you cannot see the dot. Your brain automatically "fills in" the blind spot for you – taking its cue from what it sees surrounding the blind spot. This is why you just can’t see a blind spot – even though it is right there, all the time!

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