The blind spot is caused when light falls on an area of the retina without photoreceptors. How big is this area on the retina? Here is one way to find out the horizontal diameter of the blind spot.

1. Make a tester by marking + on the far right side of a piece of notebook paper.
3. Hold the tester 500 mm (0.5 m or 50 cm) in front of your eye. (It may help to have someone help you.)
4. Close your right eye and look at the + with your left eye.
5. Place a pencil eraser on the far left side of the tester.
6. Slowly move the pencil eraser to the right.
7. When the eraser disappears, mark this location on the tester. Call this point "A."
8. Continue moving the eraser to the right until it reappears. Mark this location on the tester. Call this point "B."
9. Repeat the measurements until you are confident that they are accurate.
10. Measure the distance between the spots where the eraser disappeared and reappeared.

To calculate the width of your blind spot on your retina, let's assume that 1) the back of your eye is flat and 2) the distance from the lens of your eye to the retina is 17 mm. We will ignore the distance from the cornea to the lens.

With the simple geometry of similar triangles, we can calculate the size of the blind spot because triangle ABC is similar to triangle CDE. So, the proportions of the lines will be similar.

When I did this experiment, the measured distance between point A and point B was 46 mm. Inserting 46 mm into the equation, the blind spot on my retina has a diameter of 1.56 mm.

 Set up Example Calculations

Further questions:

• How would you measure the VERTICAL diameter of the blind spot?
• How would you measure the SHAPE of the blind spot?

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