Sometimes a person cannot clearly see objects close up or far away. To correct t

Question

Sometimes a person cannot clearly see objects close up or far away. To correct this type of vision, bifocals are often used. The top half of the lens is used to view distant objects and the bottom half of the lens is used to view objects close to the eye. A person can clearly see objects only if they are located between 35 cm and 200 cm away from her eyes. Bifocal lenses are used to correct her vision.

What focal length lens (in cm) should be used in the bottom half of the lens to allow her to clearly see objects 25 cm away?

What focal length lens (in cm) should be used in the top half of the lens to allow her to clearly see distant objects?

Explanation / Answer

so we want to map 25cm to 35 cm

1/25 + 1/-35 = 1/f

f=87.5 cm

b)

now we want infinity to make an image at -200

so f = -200 cm

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