Suppose a person who has a far point of 84.4 cm is trying to view a distant obje

Question

Suppose a person who has a far point of 84.4 cm is trying to view a distant object. What is the focal length (with correct sign) of a lens that would take a distant object and make an image on the same side of the lens as the object a distance 84.4 cm from the lens?

Lenses are prescribed in terms of their refractive power, which is expressed in terms of diopters (see the text or your favorite search engine for the definition of a diopter). What is the refractive power of this lens in terms of diopters? (do not enter units.)

Explanation / Answer

I know that the object distance is 84.4cm. I think the image distance is -50cm because the object is on the same side as the object which would make it a virtual image and virtual images have a neg number.

(1/od)+(1/id)=(1/f)

(1/84.4cm)+(-1/50)=(1/f)

-.0082=(1/f)

(1/f)=-.0082

f=(1/-.006013986)

f=-121.95

So the focal length I got was -121.95cm.

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