Question

When a person holds a magnifying glass 8.0 cm away from an object that is 2.0 cm wide, she sees a virtual image of the object, which is 6.0 cm wide. What is the focal length of the magnifying glass? Provide a drawing using the ray tracing rules.

Answer 1

Magnification factor M = 6/2 = 3

and M = - v/u

where v is the image distance and u is the object distance.

=> v = - 3u = - 3(8) = - 24 cm

now use the lens formula

1/f = 1/v + 1/u

=> 1/f = - 1/24 + 1/8

=> f = 12 cm.

this is the focal length of the magnifying glass. The image formed will be virtual, magnified and upright (not inverted).