(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A converging lens and a diverging lens, separated by a distance of30 cm, are used in combination. The converging lens has a focal length of15 cm. The diverging lens is of unknown focal length. An object is placed19 cmin front of the converging lens; the final image is virtual and is formed12 cmbefore the diverging lens. What is the focal length of the diverging lens?

2. Relevant equations

I know for sure I'm suppose to use the equations:

1/f1 = 1/p1 + 1/q1

1/f2 = 1/p2 + 1/q2

p2 = s - q1

3. The attempt at a solution

Here's what I tried to do. I understand that I'm suppose to do one lens at a time:

1/q1 = 1/f1 - 1/p1 = 1/15cm - 1/19cm = 71.25cm

p2 = s - q1 = 30cm - 71.25cm = -41.25cm

f2 = 1/q2 + 1/p2 = 16.923cm

I tried changing signs of each number part, but I'm still not getting the right answer. I tried reading class notes and textbook and I still can't figure out what I did wrong, or if there's something I'm missing

There's another similar problem to this I'm having difficulty with (which adds height), but I think I'll understand that too once I get this.

Any help would be greatly appreciated. :)

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# Homework Help: Combinination of two lenses, converging and diverging

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