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Retinal Blind Spots

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Author: Cy Ashley Webb

The lens of the eye bends light so that the light can enter the eye through the pupil. After entering the eye, light passes through the vitreous body – which most people think of as the eyeball. The retina is a membranous tissue lining the inside of vitreous body.

Millions of light-sensing nerve cells called rods and cones are found in the retina. Named for their unusual shapes, these rods and cones convert light into chemical impulses which are transmitted to the brain through the optic nerve. The optic nerve is coupled to the far side of the retina, just opposite the pupil. A tiny blind spot exists just where the optic nerve joins the retina because there are no rods or cones here.

Problem:

The goal of this experiment is to explore the limitations of sight and learn how the eye works.

Materials:

  • A pen or marker
  • Card stock
  • A yardstick

Procedure:

Experiment#1

  1. Cut out the image below and blue it to cardstock.
  2. With your arm completely extended, hold the card at eye level about an arm's length away. You can prop the card up on a yardstick to steady it. The yardstick should be roughly parallel to the floor, with one end touching your cheek. The “X” on the card should be on your right-hand side.
  3. Close your right eye. Look at the “X” with your left eye. Stay focused on the “X,” but also be aware of the dot.
  4. Slowly move the card toward you, staying focused on the “X.” As you slowly move the card, you will find a region where the dot disappears and reappears. You have found your blind spot.
  5. Repeat steps 3 and 4 with your left eye closed and your right eye focused on the dot. Can you identify a distance in which the “X” disappears?

Experiment#2

Sometimes our brain plays tricks on us to compensate for this blind spot.

  1. Cut out the image above and glue it to cardstock.Using the card just as you did in the first experiment, close your right eye. Hold up the card, balancing it on your yardstick.

  2. Focus your left eye on the “+” and slowly move the card towards you. The empty space between the lines will disappear when you reach your blind spot because your brain is compensating for the lack of an image.
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