X Marks the Blind Spot

X Marks the Blind Spot

Human Body | 10 minutes

In this activity, students will learn about how their eyes and brains work together to trick them.

Materials Needed

  • 3×5 index cards

  • Black marker

  • Ruler or straight edge for drawing a line

 

Steps:

  1. Draw a bold black dot near the middle of one end of a blank index card.

  2. Draw a bold black X near the middle of the other end.

  3. With the X on the right side of the card, hold the card in front of you at arm’s length.

  4. Close your right eye and look straight at the X with just your left eye.  Can you also see the dot? When you look one way and see something in another direction, you are using your peripheral vision.

  5. Focus on the X, but notice what the dot does as you bring the card close to you.  The dot will disappear, but as you keep moving closer it will show up again.  Experiment a little to narrow down the exact spot where the dot disappears.

  6. Try it again, but this time close your left eye and focus on the dot with your right.  Does the same thing happen with the X?

  7. Turn the card 90 degrees, and try it again.  What happens?  Turn it another 45 degrees and see what happens.

  8. Now draw a straight line from one edge of the card to the other through the dot and the X. Turn to card horizontally like the first time you tried this, close one eye, and try it again.  Notice that even though the dot or the X disappeared, the line kept going right through that spot.

Explanation:

Did you know that there is a part of your eye that can’t see?  Light travels through the opening in your eye (pupil) to the retina, which is like a projection screen in the back of your eye. It is full of light receptors that let you see. There is one spot on your retina that doesn’t have any of those cells. That spot is where your optic nerve passes through. So, when that is lined up just right, you can’t see the dot or you can’t see the X. When the line you drew passed through your blind spot, your brain automatically filled in the missing information to make it continuous.  That’s why you don’t notice your blind spot in your day to day life- Your brain just fills it in based on the image it sees around it.

Fun Fact:

An octopus has no blind spot! Their eyes are constructed in a way that makes their optic nerve never interrupt their retina.