# Focal length

1. Apr 3, 2016

### Sall1230

1. The problem statement, all variables and given/known data
" A converging lens has a focal length f. The object location at magnification equal to -1 is? "

2. Relevant equations
m = -S'/S

3. The attempt at a solution
Here is my solution:

M= -1 , S=??
M= -S'/S
-1 = -S'/S
S= -S'/-1
S=S'
And from case 2 that says if an object is placed at 2F the image will be inverted, real , and have the same size as the object.
And from the last sentence ( backed up with my conclusion of S=S' ) the object is placed at 2F.

2. Apr 3, 2016

### TSny

Your answer is correct if by 2F you mean the object should be placed at a distance 2f from the lens, where f is the focal length of the lens. The symbol F is usually used to label a focal point, so 2F doesn't have much meaning.

I'm not familiar with the "case 2" statement which apparently gives the answer immediately.

For practice, see if you can get the answer without "case 2". Use the lens equation 1/S + 1/S' = 1/f for the case where S = S'.