Question

If you are given a magnifying glass. What two methods could you use to find its focal length?

Answer 1

(1) measure the distance required to form an image of a very distance source (like the sun)

(2) Use an optical bench and form the image of a light source. Measure both do and di and use the equation
1/do + 1/di = 1/f

These methods only work for positive lenses. If the lens is diverging, use the real image from another lens and place it on the opposite side of the diverging lens, so that it can form a real image. Then use the lens equation. f will turn out to be negative.

Answer 2

Ignore my last paragraph about diverging lenses. A magnifying lens (the kind you were asked about) is never diverging.