# Homework Help: Final image from two lenses

1. Aug 4, 2012

### mrlevis

1. The problem statement, all variables and given/known data

S is a lighted object, height=1.
what is the final image that is created by the system?
2. Relevant equations

3. The attempt at a solution
i just dont understand how do i find the location of a virtual image created by a negative lens when the rays converge to a point beyond its focal point

thnks for helpers.

2. Aug 4, 2012

### Staff: Mentor

Why can't you just use the thin lens formula?

3. Aug 4, 2012

### mrlevis

you mean this?:
1/f=1/U+1/V

i used this for the lens 1, and found that V, image distance, is 12, which means it is located 6 to the negative's right, 2 after the focal point.

So how can i use the equation for the second lens? U is supposed to be an object, not converging rays.

4. Aug 4, 2012

### Staff: Mentor

All good.
The image from the first lens becomes the object for the second. But since it's past the second lens, it must be treated as a virtual object. Just use the usual sign convention: If an object before the lens has a positive object distance, one after the lens would have a negative object distance.

5. Aug 5, 2012

### mrlevis

oh... you right.
i dont know, for some reason i didnt think it would work.

but can you explain why is that? is there a simple proof to it?

6. Aug 5, 2012

### Staff: Mentor

I suggest that you review the derivation of the thin lens formula.

7. Aug 5, 2012

### mrlevis

ok, thanks for help