At 20 feet, the smallest letters that a child can read is the 20/60 line. You have the child walk toward the eye chart. How far does she walk before she can see the 20/20 line?
- 15 feet
- 7 feet
- 13 feet
- 5 feet
explanation
A 20/20 letter subtends a visual angle of 5 minutes of arc when viewed at 20 feet. As you move close to an object, it subtends a larger visual angle. A 20/60 letter subtends 3× the visual angle of a 20/20 letter. Using similar triangles,
15 arc min/5 arc min = 20 feet /6.7 feet.
Therefore, the child must advance 13 feet to be 7 feet from the eye chart to see the letters
