Description: Students will make a simple prop and use it to find their blind spot
Purpose: To locate and identify the blind spot
Length of Activity: 20 minutes
- One 3 x 5 inch card (or other stiff paper) per student.
- Black markers.
- 1 ruler per student.
1. Students should be instructed to make a dot and an X on the white side of the index card as pictured.
2. They should then hold the card so the X is on the right side and raise it to eye level about an arm's length away.
3. Have students close their right eye.
4. Student should look directly at the X with their left eye only. They should note that they can also see the dot, but should not focus on it.
5. While looking at the X, and keeping an awareness of the dot, have students bring the cards slowly towards their faces. At some point they should be aware that the dot has disappeared and then reappeared.
6. Now have students repeat but this time close their left eyes. They should use their right eyes to look at the dot while keeping aware of (but not looking directly at) the X. This time the X will disappear and then reappear as the card is slowly brought towards their faces.
7. Now have students take their markers and ruler to draw a straight line through the center of both the dot and the X.
8. Repeat the activity. Note that this time the line seems to be continuous, with no gap, even as the X or dot disappears.
What’s Going On?
At the back of your eye is the retina. Your retina is made up of light-sensitive cells which send messages to your brain about what you see. Everyone has a spot in their retina where the optic nerve connects. In this area there are no light-sensitive cells so this part of your retina can’t see. We call this the blind spot. The point at which the mark on the card disappears is where your blind spot is.
When you draw a line through the dot and X you set up an optical illusion. The brain knows that a line is there and fills in the gap, even as it loses sight of the dot or X.