Optics: Thin Lenses - Determining Focal Length

[genevievelily]

Homework Statement

In the photo (which I cannot post now, but it is a magnifying glass, and a dollar bill) the images through the lens appears twice the size and it is at a distance of 3.0cm from the lens of the magnifying glass. Determine focal length.

Homework Equations

1. -di/do=m
2. 1/f=1/di+1/do

The Attempt at a Solution

Since twice the size m will be 2. Using equation 1:

di=-m*do
= -2*3.00cm
= -6.00cm

This makes sense since image is upright and virtual, and in lenses virtual images are negative.

When I calculate focal length,

1/f=1/-6+1/3
f=6

But this answer is wrong the answer is 3. What am I doing wrong or what do I not understand?

Thanks!

 

[TSny]

Homework Statement

In the photo...the images through the lens appears twice the size and it is at a distance of 3.0cm from the lens of the magnifying glass. Determine focal length.

Perhaps the word "it" here refers to the image rather than the object.

 

[Basic_Physics]

The answer is clearly wrong. If the object was placed on the suggested focal point, 3 cm, the image would be real and located at infinity.

 

[TSny]

You will get the right answer if you interpret the distance of 3.0 cm as the magnitude of the distance from the lens to the image.