A common method to solve for the image formed due to a two lens system consisting of two thin-lenses separated by a distance is:
- Locate the (intermediate) image formed by the first lens, ignoring the second one
- Use this image as the object for the second lens to get the final image
However, how do we solve for the final image if the intermediate image is formed beyond the second lens? My textbook follows the Cartesian sign convention for optics. So, according to the book, to solve for the final image, we take the object distance for the second lens as positive instead of negative.
However, what is physically the reason for making this change of sign? Why does the lens formula even work in this case?