Blind Spot

Biology:	The Eye, Blind Spots
Grade Range:	Elementary School, Middle School
Format:	Hands-on

This is a good demonstration for a science night, or when you have groups rotating through tables. It is easy for the students to do, and the concept is one that they can easily try at home to show their family and friends!

Materials

• Dot and Cross papers
• Cardboard Tubes

Safety Precautions

Please read the General Safety section of the Demonstration Safety page before performing this demonstration.

Demonstration

1. Give each student a cardboard tube and a sheet with a dot and cross. Have them place the paper down on a table or surface in front of them, with the dot on their left side and cross on their right side.
2. Explain that they will be finding their blind spots, and give them the following instructions:
   1. Close their right eye, or cover it with their right hand. Hold the tube up to their left eye.
   2. While looking at the dot, slowly move closer to the table. At some point, the cross will disappear while they are looking at the dot!
   3. When they see the cross disappear, they can then try it with their right eye. Make sure they flip the paper around when they try it!

Why This Works

In the back of the eye is an organ called the retina, which contains a layer of very specialized cells that can sense visible light. When light strike one of these cells, the light is absorbed. Then, a signal is passed to the vision center of the brain by way of nerve cells, and the signal is translated as an image. The nerve cells which connect the retina to the brain are collected together at one spot in the back of the eye, and they form the optic nerve. This spot has no sensory cells of its own, however, and thus light which falls on it is not detected. This is the location of the blind spot!

Normally, we don’t notice we have a blind spot for several reasons. First, the blind spot is always in our peripheral vision. That means that it is in a spot in our vision that we are not focusing on, so it is never going to interfere with what we are looking at directly. Secondly, our eyes are constantly on the move. We are always looking at new things and changing focus, so nothing remains in the blind spot for very long, which makes it hard to notice. Lastly, our eyes compensate for each other. Our eyes work together to create a full image, so what falls on the blind spot in one eye will be visible to the other. All of these factors come into play for this demonstration: by covering one eye, we force the other eye to work alone. By staring at the dot, we limit how much the eye is moving. And since we are paying attention to our peripheral vision, we notice our blind spot!

Note: Sometimes a student, especially younger ones, will have a hard time with this demo and can’t find their blind spot. Either have them try it again, move the paper slower, or just don’t make them do it.

Additional Information

• This pairs well with the Temporal Tops.