# Moving an object and screen to find the focal length of a lens

1. May 12, 2014

### nofootcanman

1. The problem statement, all variables and given/known data
A convex lens is used to project the image of an object on a screen with a magnification m1 . Leaving the lens fixed, the object is moved along the optical axis
by a distance Δs and the screen is moved until the image is located again. If the new
value of the magnification is m2 , Show that the focal lenghth $\frac{Δs}{\frac{1}{m_{2}}-\frac{1}{m_{2}}}$

2. Relevant equations
s'/s=m
1/s+1/s'=1/f
Where s is the position of the object and s' is that of the image/screen

3. The attempt at a solution
I used s1=s2+Δs and attempted to rearrange but got nowhere really.. any suggestions?

2. May 12, 2014

### Staff: Mentor

Please show your work then, so we can see what went wrong.