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A farsighted person cannot focus distinctly on objects closer than 120 cm. The lens that will permit him to read from a distance of 40 cm will have a focal length of:

a) 20 cm
b) 30 cm
c) 40 cm
d) 60 cm

User MST
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1 Answer

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Final answer:

For a farsighted person who cannot focus on objects closer than 120 cm to read at a distance of 40 cm, the required focal length of a corrective lens is 60 cm.

Therefore, the correct answer is (d) 60 cm.

Step-by-step explanation:

The subject of this question is Physics, specifically in the area of geometrical optics and the correction of vision problems. The grade level of this question is High School. A farsighted person who cannot focus on objects closer than 120 cm needs a corrective lens to see clearly at a closer distance, such as 40 cm. To determine the focal length of the lens required, we can use the lens formula:

1/f = 1/do + 1/di

Where:

  • f is the focal length of the lens
  • do is the object distance from the lens
  • di is the image distance, which in this case is the near point distance of the farsighted person

Considering that for a corrective lens, do will be negative when the object is virtual (on the same side of the lens as the viewer), and di is positive (on the opposite side of the lens from the viewer), the formula becomes:

1/f = -1/40 + 1/120

After solving this equation, we find that the focal length f is 60 cm.

User Barmic
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