• Students will identify the structures of the eye.
• Students will describe how its structure causes a blind spot.
• Students will solve unknowns for similar triangles to estimate the size of their blind spot.

This lesson shows students their blind spot, which is the part in the retina of your eye where the optic nerve connects and there are no light sensitive cells. Students will estimate the size of their blind spot using similar triangles and test their range of vision.

Teacher Preparation

There is no setup beyond gathering materials.

Lesson Plan

Students will detect their blind spot and answer some questions about why and how they think it works. In this activity, students will estimate the size of their blind spot and explore the anatomy of the eye.

Locate Your Blind Spot Activity
Use the Finding Your Blind Spot Slide Show to follow along with the activity!

Students will mark an X and a dot 4 inches from one another on a 3-inch by 5-inch index card (slide 2).

Ask students to close one eye and stare at the X on the card with their open eye (slide 3). Have students hold the card at arm’s length and slowly bring the card closer to their eyes until the dot disappears.

Allow students 2 to 3 minutes to investigate this phenomenon. Direct the students to answer the questions in their Blind Spot Lab Notebook (slide 4).

After the activity is completed, explain the anatomy of the back of the eye using the Blind Spot Slide Show (slide 6). Everyone has a spot in their retina where the optic nerve connects to the eye. The optic nerve is a bundle of nerve fibers that carries visual messages from the retina of the eye to the vision center of your brain. Your optic nerve passes through one spot of your retina in your eye. In this spot, called the blind spot, there are no light receptors.

When you hold the card so the light from the dot falls on this spot, you cannot see the dot. The fovea is an area of the retina that is densely packed with light receptors, giving you the sharpest vision.

Next, demonstrate how the brain fills in the blind spot by drawing a line through the center of the dot and continuing through the center of the X (slide 7).

Measure the Blind Spot
Tell students to use the Blind Spot Lab Notebook to document data (slide 8). To measure the blind spot:

1. Hold the card 25 cm from your face.
2. Close your left eye.
3. Look at the X with your right.
4. Move a pen across the card until the point disappears and mark the card at that spot.
5. Repeat this process several times from slightly different angles and from the opposite side of the blind spot. When you have several marks, connect the marks to form a circle.
6. Draw a line through the center of the circle and measure it as the diameter.

To measure the blind spot with a partner:

1. Hold the card at arm’s length.
2. Have your partner measure the distance from the card to your eye.
3. Slowly move the card horizontally to the left and then to the right.
4. Note where the dot disappears and reappears.
5. Have your partner measure the distance between where the dot disappears and reappears.

Using their Blind Spot Lab Journals, ask students to record their measurements.

Measuring Your Blind Spot Activity
Students will use their measurements to estimate the size of their blind spots. The distance from the lenses to the retina is about 2 cm.

Students will use 2 cm in two ways: First, the measurement is used to calculate the similar triangles that will give them an approximation of the size of the blind spot. Second, students will estimate if the size of their blind spot is considered too large.

1. Ask student to use 2 cm and calculate the surface area of a sphere.
2. Use half of the estimation as an approximation of the entire retina.
3. Encourage students to answer the post-activity questions in the Blind Spot Lab Journal.

Closing
At the back of your eye is the retina. Your retina is covered in light-sensitive cells, which send messages to your brain about what you see. Everyone has a spot in the retina where the optic nerve connects. In this area, there are no light sensitive cells, so this part of your retina cannot see. We call this the blind spot. Your blind spot is not usually noticeable because your two eyes work together to cover up each eye’s blind spot.

The Sanford Connection
The Sanford Lorraine Cross Award honors a researcher or research team for breakthrough innovations that directly affect global health. Jean Bennett, MD, PhD, and Katherine High, MD, developed a gene therapy treatment for blindness and won the inaugural Lorraine Cross Award in 2018. Learn more here.

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