Determining the object distance from a magnifying glass

Homework Statement

A magnifying glass has a focal length of 8.0 cm. How close to an object do you need to place the magnifying glass so that the distance between the image and the magnifying glass is the same as the focal length? What type of image is this and what is its magnification?

Homework Equations

I am thinking that you use the Lens/Mirror equation

1/focal length = 1/object distance + 1/image distance

The Attempt at a Solution

I am unsure of how to do this because I am after object distance. So what I did was 1/object distance = 1/focal length - 1/image distance. But when I do that they just cancel because focal length and image distance are supposed to be the same thing. Once I figure that out I can handle the image type and magnification.

Thanks, ChuggZ

PhysicoRaj
Gold Member
What you have done is right. Don't stop just because they cancel out, continue and solve for what you want.

ehild
Homework Helper

The Attempt at a Solution

I am unsure of how to do this because I am after object distance. So what I did was 1/object distance = 1/focal length - 1/image distance. But when I do that they just cancel because focal length and image distance are supposed to be the same thing. Once I figure that out I can handle the image type and magnification.

The image can appear at both sides of the lens. You know only the distance of the image from the lens, but the sign of the image distance can be both positive and negative.

ehild