# Help with ambiguous question

1. Nov 14, 2009

### KBriggs

1. The problem statement, all variables and given/known data
Consider a positive thin lens L1. Using a ray diagram, show that if a second thin lense L2 is placed at the focal point of L1, the magnification does not change.

2. Relevant equations
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3. The attempt at a solution

First of all, the magnification does not change relative to what? Do they mean that the image will be the same size as if L2 was not there? Or does it mean that the magnification will be identically 1?

Either way, I drew a ray diagram for the case of an object outside the focal point of L1, using 2cm for both L2 and L1 focal lenghts. I drew a ray parallel to the optical axis into L1, which them goes to the focal point of L1 = the centre of L2, and therefore is undeviated by L2. The second ray I drew was through the focal point of L1, which emerges from L1 parallel to the optical axis, and therefore goes through the focal point of L2. The intersection gave the image, which neither had a magnification of 1, nor was it the same size as the image formed by L1 alone.

So, am I misinterpreting the question, or using the wrong rays, or what?

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