![]() ![]() ![]() Where s is the size of the blind spot on your retina (in cm), d is the diameter of the blind spot on the card, and D is the distance from your eye to the card (in the examples above, 10 in or the length of your arm, roughly 2–2.5 feet (60–75 cm). ![]() The simple equation for this calculation is Similar triangles can then be used to calculate the size of the blind spot on your retina. Light travels in a straight line through the pupil to the retina. In such a model, the pupil is 0.78 in (2 cm) from the retina. In our simple model, we are assuming that the eye behaves like a pinhole camera, with the pupil as the pinhole. Have your partner measure the distance between the two places where the dot disappears and reappears. Slowly move the card horizontally left and right, and note where the cross disappears and reappears. Have your partner measure the distance from the card to your eye. Measure the size of your blind spot with a partner: Then you can measure the diameter of the blind spot on the card (see equation below). Use the pen to trace the shape and size of your blind spot on the card. Mark the places where the pen point disappears. Move a pen across the card until the point of the pen disappears in your blind spot. (It's helpful to use a meter stick or ruler to measure this distance you'll need it to calculate the size of your blind spot.)Ĭlose your left eye and look directly at the cross with your right eye. Hold the card about 10 inches from your face. Measure the size of your blind spot without a partner: Take a new card and mark a cross near the left edge of a 3 × 5 card. This is why you don’t notice the blind spot in your day-to-day observations of the world. Your brain automatically “fills in” the blind spot with a simple extrapolation of the image surrounding the blind spot. Notice that when the dot disappears, the line appears to be continuous, without a gap where the dot used to be. Here are a few variations of this activity that you might try.ĭraw a straight line across the card, from one edge to the other, through the center of the cross and the dot, and try again. ![]()
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