System of two lenses

1. May 24, 2012

bob778111

1. The problem statement, all variables and given/known data
I don't have the exact problem, but I have a sample that I made demonstrating my question:

There are two lens, the first of focal length 20cm and the second of focal length 25cm. the image is located 15cm in front of the first lens, and the distance between the two lens is 70cm.
Find the distance of the image of the second lens, and the magnification.

2. Relevant equations
1/f=1/di+1/do
m=-di/do

3. The attempt at a solution

after calculating using 1/20=1/15+1/di, the di comes out to be -60cm.

70-(-60)=130=position of object for second lens

1/25=1/130+1/di, the di is ~30.95

magnification of first lens: -(-60/15)=4
magnification of second lens: -(30/130)=-3/13

magnification of both=-3/13*4=-12/13

But my question is: the di of the first lens came out to be negative, signifying that the image is on the same side as the object, and is virtual. Do i use the 1/f=1/di+1/do equation one more time through the first lens with positive 60 because it goes through the first lens again to get to the second lens, or do I use it directly through the second lens? Thanks for any help

2. May 27, 2012

Simon Bridge

No - what matters is the path the light travels and light does not come from the virtual image - that's why it is virtual.