# Homework Help: Physics Homework Help Needed: Optics-Lens

1. Mar 24, 2005

### Physicsme

Physics Homework Help Needed: Optics--Lens

Here's the physics problem that I need help:

A converging and diverging lens are placed 5.0 cm apart, and an object O is placed 5.0 cm from the first lens. Each lens has a 5.0 focal length. What happens to the image position if the lenses are interchanged?

Here's How I solved it:
My plan was to find the image of the object going through the first lens and then use that image as the object for the second lens. Interestingly, I kept getting the final image as 1/0, which is undefined? My first instinct was that there's something wrong with the problem!

So I went to a physics teacher for help. He said that when you have an object being at the position of the focal length, there's NO converging. He asked if my professor lectured on infinite conjugal. I said "no", I've never heard of it.

So in order to solve this problem, what do you do? Do you use limit? Do you neglect the image of the object going through the first lens and just use the object as it is going through the second lens?

I need help...I don't want the answer...I just want to know how you'd go through solving this problem...