Inserting thick lenses into a thin lens system and deducing values I have two positive thin lenses that are separated by a distance of $5 cm$. The focal lengths of the lenses are $F_1 = 10 cm$ and $F_2 = 20 cm$. I placed an object 2 cm to the left of the front focal point and calculated the image by using the equation $\dfrac{1}{f} = \dfrac{1}{s_o} + \dfrac{1}{s_i}$ twice, so that the image of the first lens became the object of the second lens.
My system looks like this:

(credit to www.livephysics.com)
I am then told to insert two thick bi-convex lenses of $10 cm$ and $20 cm$ effective focal lengths into the system instead of the thin optical lenses, so that I get the same image with the same object $2 cm$ to the left of the front focal point. These thick lenses have all of the typical information  available that one would find in a lens catalogue (radius of curvature, principal plane distances, refractive index, etc.), and I can provide this information if requested.
After insertion of the thick lenses, my system looks like this:

(From Optics, Fifth Edition, by Hecht.)
(Note that $d$ in the thick lens picture above is different to the distance that I am describing below.)
I'm now trying to find the distance between the back surface of the first lens and the front surface of the back lens.
I have two problems:


*

*I'm not sure that I'm correctly interpreting what is meant by inserting the thick lenses, so that one gets the same image with the same object $2 cm$ to the left of the front focal point.

*Despite a tremendous amount of research, I don't understand how it is possible to infer the the distance between the two thick lenses that were inserted instead of the thin lenses. I wondered if I was just misunderstanding what is exactly meant by "inserting" thick lenses instead of thin lenses in a system, but I have looked through a lot of optics resources and found nothing that indicates that the lenses must be "inserted" in a specific way that allows for deduction of the distance. I am told that the distance is somewhere between $4 - 5cm$, but I don't understand how such a thing can be calculated. Since this value is so close to the original thin lens distance of $5cm$, this leads me to believe that I am completely misunderstanding something about the nature of thick lens systems and how they are "inserted" into a thin lens system. Could it be that, to "insert" a thick lens instead of a thin lens means to align the secondary principal plane of the first thick lens with the first thin lens, and align the primary principal plane of the second thick lens with the second thin lens, so that the distance between the secondary principal plane of the first lens and the primary principal plane of the second lens is $5 cm$?
I would greatly appreciate it if people could please take the time to clarify this.
 A: At this level of approximation (parallax approximation/Gaussian optics), a thick lens is nothing but a thin lens with rays "skipping" or "teleporting" through the distance between the two principal planes.
So if you use a thick lens with the same focal distance as the thin one, just place its front principal plane where the thin lens used to be. Then for the following components, measure all the same distances you had for the thin lens, but measure them from the back principal plane this time. In effect, you're just adding the thickness of the lens to the whole system - nothing more and nothing less.
The thickness is just the distance between the principal planes that is getting "skipped over". If the thickness is positive, the rear principal plane is behind the front one and the rays skip forward. If the thickness is negative, the rear principal plane is in front of the front one (maybe a bit confusing) and the rays jump back.
As for what the question meant by "same image", I don't know but I'd guess it means same orientation, same magnification.
This sounds a bit like a homework question so I won't post any specific numbers, but hopefully I helped you understand the nature of the thick lens to see it's almost as simple as dealing with thin ones.
EDIT:
Here's what transforming a thin lens into a thick lens means. It's still the same lens, just has a gap in between. Where the rays disappear is the front principal plane, where they reappear is the rear principal plane.

