Magnifying glass: What is the focal length of a magnifying glass that produces an angular magnification of 7.0 x when used by a person with a near point at 28 cm?

Question

Quizplus · Accepted Answer

We use the formula for angular magnification:
m = -f/ (d - f)
where f is the focal length of the magnifying glass, d is the distance between the lens and the object, and m is the angular magnification obtained. We assume that the person can see the image formed by the magnifying glass at the normal near point of 25 cm, which is actually 28 cm for this person.
So, 7 = -f / (28 - f)
Solving for f, we get f = 4 cm. Thus, the focal length of the magnifying glass is 4 cm. Therefore, the best option is (C).

Magnifying Glass: What Is the Focal Length of a Magnifying