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## Homework Statement

Two 28.0-cm-focal-length converging lenses are placed 16.5 cm apart. An object is placed 36.0 cm in front of one lens. Where will the final image be formed by the second lens?

## Homework Equations

1/d

_{o}+ 1/d

_{i}= 1/f

## The Attempt at a Solution

for the first lens:

1/d

_{o}+ 1/d

_{i}= 1/f

1/d

_{i}=1/28 - 1/36 = 8/1000

d

_{i}= 125cm

This is where I get confused I used 125cm as my d

_{o2}but the solution tells me I have to do 16.5cm - 125cm. My question is since the image due to lens 1 is 125cm away from lens 1, why would i have to subtract the lens separation in order to get d

_{o2}? I know that d

_{o2}is a negative number but not why i have to take into account the lens seperation.

Thanks for you help