# Convex Lens and Magnification

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1. Apr 7, 2015

### Sylvia

1. The problem statement, all variables and given/known data
You wish to obtain a magnification of -2 from a convex lens of focal length f. The only possible solution is to:

A) place a virtual object at a distance 2f/3 from the lens.
B) place a virtual object at a distance 5f/3 from the lens.
C) place a real object at a distance 3f/2 from the lens.
D) place a real object at a distance 2f/3 from the lens.
E) place a virtual object at a distance 3f/2 from the lens.

2. Relevant equations
(1/do) + (1/di) = (1/f)
m = -di/do
f is negative because it is a convex lens

3. The attempt at a solution
I'm not sure how to factor in focal length, I am unsure how to get the relationship between object distance and focal length

2. Apr 8, 2015

### Estifericate

Sylvia, you can attempt this quetion by eliminating approach.
Here's what you should consider,
First try by using real objects.
Note that for magnification of -2 you should have an inverted image.
We kniw for object between f and o , the image is virtual and erect hence,
Option D is ruled out.
Now considering option C
The objet will be between f and 2f
Hence image will be real and enlarged.
And also by putting magnification ( -v/u = -2) you can get your answer.
Hence option C seems correct to me.

If you want further solution letme know.

3. Apr 8, 2015

### ehild

The focal length of convex lenses is positive.
You know how magnification defined. So you can express do in terms of di and substitute into the equation (1/do) + (1/di) = (1/f). Solve for do in terms of f.

4. Apr 8, 2015

### Sylvia

Thank you both very much, I understand it now!