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In the explanation to this question, it states "the 140cm focal point is where the image needs to be located". Can someone explains what this means?

A certain farsighted person cannot focus on objects closer to his eyes than 140cm. What focal length eyeglass lenses are needed in order to focus on a newspaper held at 35cm from the person's eyes, if the glasses are worn 2cm from his eyes?

2cm

27cm

47cm

28cm

43cm

Explanation:

First we need the object and image distances away from the eyeglass lenses. Here, the newspaper is the object and the 140cm focal point is where the image needs to be located. Find the distance from the object to the lens, and the distance of the image to the lens, by subtracting out the distance from the lens to the eye.

dol=doe−dle=35cm−2cm=33cm

dil=die−dle=140cm−2cm=138cm

Now apply the thin lens equation to determine focal length.

1f=1di+1do

Recall that if the image is on the same side of the lens as the object, then image distance is negative.

1f=1−138+133≈0.023

f=10.023≈43cm

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