Blind Spot – Lens Optics
Sheets of paper (A5 ish will do), pen (or printer)
Draw and “X” and an “O” on the sheet of paper about 6cm apart as shown. (or print the image below)
Gets students to cover their left eye with their hand and hold the paper at arms length in their free hand. Get them to stare at whichever letter is on the left of the paper and SLOWLY bring it towards themselves – still staring at the left hand letter. As they do the right hand letter will disappear and eventually reappear as it gets close enough to their face.
This illustrates (as it works the same for the other eye) that both your eyes have a blind spot that you don’t normally notice. Why because you have 2 eyes and your brain is smart enough to combine the images so you don’t notice the blind spot. But what about when you close one eye? You don’t ‘see’ the black patch indicated by a blind spot! Thats’s because your brain ‘samples’ the colour of what surrounds your blind spot and uses that to ‘make up’ what is inside it! But why do we have a blindspot at all? The retina at the back of your eye (the black pupil you see when you look at someone’s eye) is actually a surface covered in millions of tiny cells. These are all sensetive to light, as they contain a pigment called an ‘opsin’ that changes shape when you shine light on it. So when some light falls on the back of your eye, these shape chanes send an electrical signal to you brain that your brain translated into an image. The blind spot is caused by the ‘cable’ that runs from your eye to your brain, covering part of the retina and blocking light – so you can’t see anything behind it – hence a ‘blind spot’ . It’s a bit like it’s casting a shadow on your retina, but our brain has learned to work around it and let us see anyway – thats why we don’t notice! You can see it labelled the ‘optic disc’ on the diagram above