Find Your Blind Spot!
Did you know that you have a blind spot in your eye? In fact, every person has an unavoidable blind spot in each of their eyes. This doesn't mean that you see a constant black spot in your field of vision though. Normally you don't notice these blind spots at all, however there are some ways how you can make these blind spots visible. This activity will show you how to find them. Can you find them or not?
- Cardstock paper
- Ruler or measuring tape
- Marker or pen
- Cut a 2-inch high and 5-inch long piece of paper from the cardstock.
- On the left side of the paper strip, draw a shape such as a circle, heart or plus sign. The shape should not be much wider than half an inch.
- On the right side of the paper strip draw another shape. It should also not be much wider than half an inch.
- Take the paper strip in your right hand. Hold it in the middle so you can see both shapes in each of the corners.
- Extend your arm with the paper strip at eye level. Focus your eyes on the left shape.With your eyes still focused on the left side, can you still see the shape on the right side of the paper strip?
- Slowly move your extended arm closer to your face. While moving the paper closer, keep your eyes focused on the left shape.While moving the card closer, can you still see both shapes clearly?
- Cover your left eye with your left hand. Extend the right arm with the paper strip at eye level again. Focus your right eye on the left shape.Can you see the other shape as well?
- With your left eye covered and your right eye focusing on the left shape, slowly move the paper strip closer towards your face. Keep focusing your right eye on the left shape.What happens to the shape on the right side of the paper strip while you move the paper strip closer?
- Now cover your right eye with your right hand. Extend your left arm with the paper strip and look at the right shape.Are you able to still see the left shape while focusing on the right shape with your left eye?
- Again, slowly move the piece of paper closer to you. Keep your left eye focused on the right shape.What do you notice this time?
- Dispose of the paper in your recycling bin and move all other materials to where they belong.
Did you find your blind spot? When looking at the paper strip with both eyes, you probably always saw both shapes in each corner of the paper, even when you moved your arm closer to your body. However, when you looked with your right eye only directly at the left shape you should have noticed that at some point the right shape disappeared as you moved the paper closer to your face. Of course, the shape did not really disappear because it was clearly still drawn on the paper. At this position the shape just happened to be in your blind spot. This blind spot results from the optical nerve fibers that pass in front of your light sensitive cells inside your eye. Where the nerve passes through the retina, there are no rods and cones that allow you to see. At this tiny spot, that is approximately the size of a pinhead, you are blind. You know that you have such a blind spot in both of our eyes as you should have seen the same happen when you looked with your left eye onto the right shape. This time, however, the left shape should have disappeared.
If you did the extra experiment with the straight line across the card, you should have noticed that the shape still disappeared but instead you saw a continuous straight line all the way to the edge of the paper. This is a great example of how your brain tries to fill in the blanks of the blind spot. Your brain noticed the straight line in the surrounding of your blind spot and just continued the line although it technically didn't see it. However, the brain couldn't notice the shape as it was fully inside the blind spot. As a result, the shape disappeared but the straight line was still visible.
Our eyes are complex organs that allow us to see objects, colors, and everything that happens around us. To be able to see, however, we need light. Usually what we see is light that reflects from objects and then enters our eyes through the pupil, which is an opening in the middle of the front of the eye. How much light gets into the eye is controlled by the iris, which is the colored part around the pupil. The iris is connected to a muscle that allows the pupil to open and close in a circular motion. Inside the eye the light lands on the retina at the back of the eye, which is a light sensitive layer of tissue. The retina has two types of light-sensing cells, the rods and the cones. Rods provide black-and-white vision in dim light whereas the cones are responsible for color vision.
When light hits the rod and cone cells, nerve impulses are triggered and sent to the brain through the optical nerve. In humans and most vertebrates, the optical nerve fibers route before the retina and pass through the retina out of the eyeball. The area where the bundled nerve fibers pass through the retina does not contain any light sensitive cells. This means that if light hits this exact spot, we won't be able to see it. Although we technically cannot see this light, our brain is able to fill in the information that we are missing based on the images surrounding our blind spot. This is the reason why we don't usually notice it.
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For Further Exploration
- On your card, draw a straight line across the card from one edge to the other. The line should go through your two shapes. Repeat the experiment. How do your results change?
- Can you measure the size of your blind spot? Cover one of your eyes and position the card at a distance where one of the shapes disappears from your vision. Move a pen across the card and mark where it disappears in your blind spot from all sides. The markings allow you to measure the size of your blind spot on the card.
- Make a new paper card but this time draw bigger shapes in each corner. Does the experiment still work?
- MS-LS1-8. Gather and synthesize information that sensory receptors respond to stimuli by sending messages to the brain for immediate behavior or storage as memories.
- Blog Post: Science Buddies: Binocular Vision Eye Science
- Ducksters: Biology for Kids: Sight and the Eye
- Healthline: Why Do I Have a Blind Spot in My Eye?
- Science Daily: We've all got a blind spot, but it can be shrunk.