# Focal Length given M and distance between obj and img

Tags:
1. Dec 3, 2017

### PhyHyped

1. The problem statement, all variables and given/known data
The distance between an object and its upright image is 28.0 cm. If the magnification is 0.550, what is the focal length of the lens that is being used to form the image?

2. Relevant equations
1/f = 1/so + 1/si
M= -si/so

3. The attempt at a solution
See included image.

I've been having a hard time with this all day any help would be amazing.

Can't seem to get the right answer.

File size:
65.7 KB
Views:
20
2. Dec 3, 2017

### lekh2003

Ok, there is one problem with your diagram. The image is on the wrong side of the lens. The object is a certain distance, s0, from the lens on the left side. The image is a certain distance, si, from the lens on the opposite side.

In your diagram, si + 0.28 m = s0. But you must remember that si is negative. Look back at your equation for magnification: m = -si/s0. The s0 is positive, so the distance for the image, si, should be negative in order to produce a positive magnification.

I hope that helped. Just try and do your same equations, but in the diagram, remember that the distance for the image in the equation is negative. Then, just list down all the equations you have, and you should see a simultaneous equation.

3. Dec 3, 2017

### PhyHyped

thanks! ill give that a try

4. Dec 3, 2017

### lekh2003

You're welcome.

5. Dec 3, 2017

### PhyHyped

Solved it! Thanks again!